A group of 36 diabetes-prone BB Wistar rats were given prophylactic insulin treatment with heat-treated bovine ultralente insulin (15 IU X kg-1 X day-1) from 50 to 142 days of age. The incidence of Type 1 (insulin-dependent) diabetes at the end of the treatment period was compared to that of 36 control animals given insulin only from the first day of glycosuria. At withdrawal of the prophylactic insulin treatment, 6 of 36 treated animals were insulin-dependent, while 15 of 36 control animals had developed diabetes (p less than 0.02). One control animal (day 153) and 2 insulin-treated animals (day 172 and 186) subsequently developed diabetes, yielding an overall diabetes incidence of 16 of 36 controls against 8 of 36 insulin treated (p less than 0.05). The finding that prophylactic insulin treatment of diabetes-prone BB Wistar rats reduced the incidence of diabetes suggests a relationship of the immune destruction of B cells to the endogenous insulin production/secretion rate.