Our recent observation indicated that overexpression of HLA-class I antigen on pancreatic beta cells is one of the features of insulin-dependent diabetes mellitus (IDDM). To analyze the effect of class I antigen overexpression, we have introduced human HLA class I (HLA-Cw3) gene into rat pancreatic beta cell lines, RINm5F. Several stable transformants with various levels of surface expression of class I antigens have been cloned. The insulin secretion of these transfectants was analyzed to evaluate the functions of beta cells. Highly negative correlation between the level of insulin secretion and that of class I expression (correlation coefficiency: r = 0.89) has been observed. These data suggest that the overexpression of HLA class I antigen itself may impair pancreatic beta cells through a nonimmune mechanism.