The destructive insulitis that develops in disease-prone NOD mice is characterized by a high frequency of IFN-gamma-producing cells in the lesion. Complete Freund's adjuvant (CFA) challenge of young, disease-prone animals inhibits the development of diabetes but does not alter the frequency with which pancreatic insulitis develops. However, the non-destructive lesions that develop in syngeneic islets transplanted to the kidney capsule of NOD mice following CFA therapy differ in cytokine production from the destructive lesions that develop in control animals transplanted with NOD islets. Non-destructive lesions are characterized by a high frequency of IL4-producing cells and a relatively low frequency of IFN-gamma-producing cells.