After a retreat to an Oregon farm, 19 of 31 college students developed an acute gastrointestinal illness. Campylobacter jejuni infection was recognized in all the ill students and caused asymptomatic infections in three others. In total, 22 (88%) of 25 students who consumed raw milk for the first time became infected as compared with none of two who had not consumed raw milk. Among ten persons who chronically consumed raw milk, none was ill, a striking difference from the 76% attack rate among the 25 acutely exposed students. The quantity of raw milk consumed was directly related to the occurrence and severity of illness. Acutely infected students showed significant rises in C jejuni—specific immunoglobulins, whereas the low antibody levels seen in unexposed persons did not rise. In contrast, acute-phase serum samples from persons with chronic exposure to raw milk showed elevated antibody levels to C jejuni. These findings indicate that chronic raw milk consumption is associated with elevated levels of C jejuni— specific serum antibodies and with immunity to symptomatic infection.