Two strains of Campylobacter jejuni ingested by 111 adult volunteers, in doses ranging from 8 x 10(2) to 2 x 10(9) organisms, caused diarrheal illnesses. Rates of infection increased with dose, but development of illness did not show a clear dose relation. Resulting illnesses with strain A3249 ranged from a few loose stools to dysentery, with an average of five diarrheal stools and a volume of 509 mL. Infection with strain 81-176 was more likely to cause illness, and these illnesses were more severe, with an average of 15 stools and 1484 mL of total stool volume. All patients had fecal leukocytes. The dysenteric nature of the illness indicates that the pathogenesis of C. jejuni infection includes tissue inflammation. Ill volunteers developed a serum antibody response to the C. jejuni group antigen and were protected from subsequent illness but not infection with the same strain.