The histopathological findings in a group of cows injected with homologous or heterologous insulin in adjuvant are described. The essential features in the most severely affected cows (including two which received only bovine insulin) were lymphocytic infiltration of the islets of Langerhans with destruction of beta cells and fibrosis. The surviving beta cells showed signs of over-activity. The findings, which are regarded as having the characteristics of an “autoimmune” response, are discussed in relation to similar pathologic findings in human juvenile diabetes of recent onset.