The invention relates to developments in the treatment of diabetes in mammals. Particularly it relates to a method of preparing a xenotransplantable porcine islet preparation capable upon xenotransplantation of producing porcine insulin in an appropriate recipient mammal, the method including or comprising the steps of: (I) harvesting the pancreas of piglets at or near full term gestation, and (ii) extracting pancreatic .beta. islet cells from the harvested pancreas wherein the islets (at least at some stage in the performance of the method) are exposed to nicotinamide. Further, the invention relates to a method of encapsulation of a xenotransplantable porcine islet preparation, and transplantation of such a preparation, or a capsule containing such a preparation, into an appropriate recipient mammal.