The diabetic syndrome in the DP-BB rat results from progressive beta-cell destruction by autoimmune responses. However, the initial events causing the autoimmune destruction of beta cells remain largely unknown. Our recent experimental results suggest that the delayed expression of a beta-cell-specific autoantigen may result in the initiation of beta-cellspecific autoimmunity. The present investigation was initiated to identify such an autoantigen. Islets were isolated from DP-BB rats of several different ages, and protein extracts from the membrane fraction of the islet preparations were immunoprecipitated with sera from diabetic DP-BB rats. We have found that a membrane-bound islet cell-specific 38 kDa autoantigen is not expressed early in the life of DP-BB rats, but is delayed-expressed by approximately 30 days of age, the time at which immunological effectors begin to recognize beta cells. In contrast, a 64 kDa islet cell protein is expressed from birth in DP-BB rats. On the basis of these observations, we suggest that delayed expression of a gene encoding for the membrane-bound islet cell-specific 38 kDa autoantigen may result in a breakdown of self-tolerance, leading to beta-cell-specific autoimmune IDDM in the BB rat. [Diabetologia (1994) 37: 460-465]