Sinclair had spent about six months investigating the Chicago meatpacking industry for Appeal to Reason, the work which inspired his novel. He intended to "set forth the breaking of human hearts by a system which exploits the labor of men and women for profit". The novel featured Jurgis Rudkus, a Lithuanian immigrant who works in a meat factory in Chicago, his teenaged wife Ona Lukoszaite, and their extended family. Sinclair portrays their mistreatment by Rudkus' employers and the wealthier elements of society. His descriptions of the unsanitary and inhumane conditions that workers suffered served to shock and galvanize readers. Jack London called Sinclair's book "the Uncle Tom's Cabin of wage slavery". Domestic and foreign purchases of American meat fell by half.
Sinclair wrote in Cosmopolitan in October 1906 about The Jungle: "I aimed at the public's heart, and by accident I hit it in the stomach." The novel brought public lobbying for Congressional legislation and government regulation of the industry, including passage of the Meat Inspection Act and the Pure Food and Drug Act. At the time, President Theodore Roosevelt characterized Sinclair as a "crackpot", writing to William Allen White, "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, but was opposed to legislation that he considered "socialist". He said, "Radical action must be taken to do away with the efforts of arrogant and selfish greed on the part of the capitalist.