The effect of repeated subcutaneous injections of homogenized rat islets on the morphology of the pancreas of the injected animals was determined. Male Wistar rats were injected with fifty homogenized islets every two weeks and were killed in groups of two to three over a twelve-month period beginning two months after the first injection of islets. The chronic injection of islet homogenates produced fibrosis of the islets in the injected animals with marked distortion of the architecture of the islets and the deposition of hemosiderin in the peri-insular tissue. In a few instances, lymphocytic infiltration was observed in the peri-insular area. The cause of this unusual islet lesion was presumably due to an antigen-antibody reaction in the islets. None of the injected animals developed either hyperglycemia or glycosuria.