Recently many investigators have attempted transplantation of isolated islets of Langerhans in an expectation of therapeutic effects in patients with diabetes. In the course of time, preservation of the islets was found to be a prerequisite for successful transplantation. In the present studies, we used our newly designed method of preserving islets, i.e., cryopreservation combined with the cultural process before and after freezing, and we examined the insulin-releasing activity of the islets and attempted transplantation of the islets so-obtained. As to the insulin-releasing activity, the islets cryopreserved after short-term culture increased insulin output as compared with those cryopreserved without culture. Damage to Langerhans' islets resulting from isolation and cryopreservation was overcome during the culture. Transplantation of Langerhans' islets cultured for 3-4 days before and after the cryopreserving process produced favorable results.