Yersinia enterocolitica biotype 1, serotype O.10K was isolated from 19 patients in the paediatric wards of a district general hospital over a period of 3 months. Fifteen cases were patients on the medical ward. Shortly afterwards, Y. enterocolitica biotype 1, serotype O.6, 30 was isolated from a further 17 patients on this ward in 1 month. The same serotypes of Y. enterocolitica were isolated from the pasteurized milk supplied to the ward. Epidemiological evidence indicated that contaminated pasteurized milk was the source of the yersinia organisms excreted by the patients.