Upton Sinclair intended to expose "the inferno of exploitation [of the typical American factory worker at the turn of the 20th Century]", but the reading public fixed on food safety as the novel's most pressing issue. Sinclair admitted his celebrity arose "not because the public cared anything about the workers, but simply because the public did not want to eat tubercular beef". Some critics have attributed this response to the characters, most of whom, including Rudkus, have unpleasant qualities. The last section, concerning a socialist rally Rudkus attended, was later disavowed by Sinclair. But his description of the meatpacking contamination captured readers' attention.
Sinclair's account of workers falling into rendering tanks and being ground along with animal parts into "Durham's Pure Leaf Lard" gripped the public. The poor working conditions, and exploitation of children and women along with men, were taken to expose the corruption in meat packing factories.
The British politician Winston Churchill praised the book in a review.
In 1933, the book became a target of the Nazi book burnings due to Sinclair's endorsement of socialism.
President Theodore Roosevelt had described Sinclair as a "crackpot" because of the writer's socialist positions. He wrote privately to journalist William Allen White, expressing doubts about the accuracy of Sinclair's claims: "I have an utter contempt for him. He is hysterical, unbalanced, and untruthful. Three-fourths of the things he said were absolute falsehoods. For some of the remainder there was only a basis of truth." After reading The Jungle, Roosevelt agreed with some of Sinclair's conclusions. The president wrote "radical action must be taken to do away with the efforts of arrogant and selfish greed on the part of the capitalist." He assigned the Labor Commissioner Charles P. Neill and social worker James Bronson Reynolds to go to Chicago to investigate some meat packing facilities.
Learning about the visit, owners had their workers thoroughly clean the factories prior to the inspection, but Neill and Reynolds were still revolted by the conditions. Their oral report to Roosevelt supported much of what Sinclair portrayed in the novel, excepting the claim of workers falling into rendering vats. Neill testified before Congress that the men had reported only "such things as showed the necessity for legislation." That year, the Bureau of Animal Industry issued a report rejecting Sinclair's most severe allegations, characterizing them as "intentionally misleading and false", "willful and deliberate misrepresentations of fact", and "utter absurdity".
Roosevelt did not release the Neill–Reynolds Report for publication. His administration submitted it directly to Congress on June 4, 1906. Public pressure led to the passage of the Meat Inspection Act and the Pure Food and Drug Act; the latter established the Bureau of Chemistry (in 1930 renamed as the Food and Drug Administration).
Sinclair rejected the legislation, which he considered an unjustified boon to large meat packers. The government (and taxpayers) would bear the costs of inspection, estimated at $30,000,000 annually. He complained about the public's misunderstanding of the point of his book in Cosmopolitan Magazine in October 1906 by saying, "I aimed at the public's heart, and by accident I hit it in the stomach."