After a one-day field trip to a Minnesota farm, 22 (45%) of 49 third-grade students and three (14%) of 21 adult chaperones developed campylobacteriosis. Campylobacter jejuni was isolated from specimens of 13 children and one asymptomatic adult. Illness was associated with the consumption of raw milk during a picnic lunch (odds ratio = 41.0, P less than .0001) and participation in hand milking of cows (odds ratio = 37.5, less than .0001). Two additional findings implicated consumption of raw milk as the vehicle for transmission. First, the odds ratio for illness among those who drank raw milk and did not milk cows was 11.8 (P less than .01), whereas the odds ratio for illness among those who milked cows but did not drink raw milk was only 1.2 (P greater than .02). Second, there was a dose-response relation between the occurrence of illness and the quantity of raw milk consumed (P less than .0001). Neither the median incubation period nor duration of illness was related to the amount of milk consumed. All persons with a stool culture positive for C. jejuni excreted the organism for at least two weeks, but less than six weeks. No secondary cases of illness were documented.