BACKGROUND The development of islet cultures is desirable for successful clinical islet transplantation. Fetal bovine serum (FBS) has been used as a supplement in islet culture medium, but it may be an unsuitable supplement due recent animal health problems. We have evaluated the use of the silk protein, sericin, derived from Bombyx mori as a replacement for FBS in islet culture medium. METHODS Twenty rat islets were cultured in medium containing either sericin or FBS, or no supplement, for 14 days, during which time viable islets were counted in order to evaluate islet survival. Insulin secretion was measured in vitro by static incubation on days 3 and 7. In vivo function of cultured islets was tested by syngeneic transplantation. The islets were evaluated histologically and immunohistochemically after culture and transplantation. RESULTS Ninety-five percent of islets were viable after culture for 14 days in culture medium supplemented with either FBS or sericin, while no islets survived beyond 7 days in culture without supplement. No significant differences in stimulated insulin secretion were noted between two groups of islets grown on supplemented media. Following transplantation, islets cultured in FBS or sericin rapidly reversed hyperglycemia and maintained normal glycemic control. Histologically, islets cultured with sericin displayed a well-preserved structure and strong insulin staining before and after transplantation. CONCLUSIONS Serum-free medium containing sericin appears to be useful for islet culture.