In the summer of 1987-1988, an outbreak of 11 cases of Yersinia enterocolitica enteritis caused by 2 serogroups (0:3, 0:6,30) occurred prompting an investigation into possible environmental sources. Symptoms were present for a mean of 9 days and occurred in 2 distinct age groups--toddlers (7) who presented with diarrhea, and young adults (4), 3 of whom presented clinically with appendicitis. In a survey of 39 randomly chosen pasteurized milk samples, 9 were positive for growth of Y. enterocolitica and 1 each for Y. fredericksenii and Y. intermedia. An association between clinical and milk isolates of Y. enterocolitica was thus sought by comparison of biogroups, serogroups, virulence markers and biochemical and outer membrane profiles. All milk isolates belonged to biogroup 1, serogroup 0:6,30. Pathogenicity studies on the 0:6,30 serogroup isolates from feces and milk were performed with 3 in-vitro tests (Ca2+ dependency, autoagglutination, & serum resistance). The human isolates were positive in most of the 3 tests whilst none of the milk isolates were positive. Outer membrane protein analysis of 0:6,30 from human and milk isolates showed similar profiles suggesting a possible association, however the environmental source of the majority of isolates (0:3) remains unknown.