On August 3, 1948, Chambers was called to testify before the House Un-American Activities Committee (HUAC). Here he gave the names of individuals he said were part of the underground "Ware group" in the late 1930s, including Alger Hiss. He thus once again named Hiss as a member of the Communist Party, but did not yet make any accusations of espionage. In subsequent HUAC sessions, Hiss testified and initially denied that he knew anyone by the name of Chambers, but on seeing him in person (and after it became clear that Chambers knew details about Hiss's life), said that he had known Chambers under the name "George Crosley". Hiss denied that he had ever been a Communist, however. Since Chambers still presented no evidence, the committee had initially been inclined to take the word of Hiss on the matter. However, committee member Richard Nixon received secret information from the FBI which had led him to pursue the issue. When it issued its report, HUAC described Hiss's testimony as "vague and evasive".
The country quickly became divided over the Hiss–Chambers issue. President Harry S Truman, not pleased with the allegation that the man who had presided over the United Nations Charter Conference was a Communist, dismissed the case as a "red herring". In the atmosphere of increasing anti-communism that would later be termed McCarthyism, many conservatives viewed the Hiss case as emblematic of what they saw as Democrats' laxity towards the danger of communist infiltration and influence in the State Department. Many liberals, in turn, saw the Hiss case as part of the desperation of the Republican party to regain the office of president, having been out of power for 16 years. Truman also issued Executive Order 9835, which initiated a program of loyalty reviews for federal employees in 1947.
Hiss filed a $75,000 libel suit against Chambers on October 8, 1948. Under pressure from Hiss's lawyers, Chambers finally retrieved his envelope of evidence and presented it to the HUAC after they subpoenaed them. It contained four notes in Alger Hiss's handwriting, sixty-five typewritten copies of State Department documents and five strips of microfilm, some of which contained photographs of State Department documents. The press came to call these the "Pumpkin Papers" referring to the fact that Chambers had briefly hidden the microfilm in a hollowed-out pumpkin. These documents indicated that Hiss knew Chambers long after mid-1936, when Hiss said he had last seen "Crosley," and also that Hiss had engaged in espionage with Chambers. Chambers explained his delay in producing this evidence as an effort to spare an old friend from more trouble than necessary. Until October 1948, Chambers had repeatedly stated that Hiss had not engaged in espionage, even when Chambers testified under oath. Chambers was forced to testify at the Hiss trials that he had committed perjury several times, which reduced his credibility in the eyes of his critics.
The five rolls of 35 mm film known as the "pumpkin papers" were thought until late 1974 to be locked in HUAC files. Independent researcher Stephen W. Salant, an economist at the University of Michigan, sued the U.S. Justice Department in 1975 when his request for access to them under the Freedom of Information Act was denied. On July 31, 1975, as a result of this lawsuit and follow-on suits filed by Peter Irons and by Alger Hiss and William Reuben, the Justice Department released copies of the "pumpkin papers" that had been used to implicate Hiss. One roll of film turned out to be totally blank due to overexposure, two others are faintly legible copies of nonclassified Navy Department documents relating to such subjects as life rafts and fire extinguishers, and the remaining two are photographs of the State Department documents introduced by the prosecution at the two Hiss trials, relating to U.S./German relations in the late 1930s.
This story, however, as reported by the NY Times in the 1970s, contains only a partial truth. The blank roll had been mentioned by Chambers in his autobiography Witness. But in addition to innocuous farm reports, etc., the documents on the other pumpkin patch microfilms also included "confidential memos sent from overseas embassies to diplomatic staff in Washington, D.C."; worse, those memos had originally been transmitted in code, which, thanks to their (presumably) having both coded originals and the translations forwarded by Hiss, the Soviets now could easily understand.
Hiss could not be tried for espionage at this time, because the evidence indicated the offense had occurred more than ten years prior to that time, and the statute of limitations for espionage was five years. Instead, Hiss was indicted for two counts of perjury relating to testimony he had given before a federal grand jury the previous December. There he had denied giving any documents to Whittaker Chambers, and testified he had not seen Chambers after mid-1936.
Hiss was tried twice for perjury. The first trial, in June 1949, ended with the jury deadlocked eight to four for conviction. In addition to Chambers's testimony, a government expert testified that other papers typed on a typewriter belonging to the Hiss family matched the secret papers produced by Chambers. An impressive array of character witnesses appeared on behalf of Hiss: two U.S. Supreme Court justices, Felix Frankfurter and Stanley Reed, former Democratic presidential nominee John W. Davis and future Democratic presidential nominee Adlai Stevenson. Chambers, on the other hand, was attacked by Hiss's attorneys as "an enemy of the Republic, a blasphemer of Christ, a disbeliever in God, with no respect for matrimony or motherhood". In the second trial, Hiss's defense produced a psychiatrist who characterized Chambers as a "psychopathic personality" and "a pathological liar".
The second trial ended in January 1950 with Hiss found guilty on both counts of perjury. He was sentenced to five years in prison.