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 β 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.