Intravenous transfusion of concanavalin A-activated splenic cells from acutely diabetic BB or diabetic BB/hooded hybrid donor rats into 6- to 36-h-old neonate recipients of diabetes-prone and -resistant rat lines induced insulitis and in some severe diabetes. These effects were observed 10-20 days after the injection of the blasts. Focal lesions of insulitis were absent in neonates killed 1 and 3 days after the blast injection but were observed in neonates killed on the 5th and 8th day. As determined by autoradiography after the injection of [3H]thymidine-labeled blasts, numerous blast cells migrated and settled in various immature lymph nodes and in the spleen within 24 h after injection. Focal mononuclear infiltrations in the islets containing labeled and unlabeled cells were again observed on the 5th and 8th day but not on the 1st and 3rd day after injection. These experiments indicate that target-specific blasts undergo a short phase of proliferation and maturation in lymphoid organs of the recipients, before initiating the autoimmune process in the pancreatic islets. They further suggest that specific immune cells rather than humoral anti-islet antibodies are more likely to play the major role in this autoimmune animal model of diabetes.