Twenty four patients with established insulin dependent diabetes treated with twice daily soluble and isophane bovine insulins were changed to equivalent doses of either purified bovine Neusulin and Neuphane (Wellcome) or purified porcine Actrapid and Monotard (Novo) insulins. After 6 months treatment the porcine group showed a 35% fall in insulin binding antibodies and a 14% reduction in insulin dosage. The group changed to purified bovine insulins showed no significant change in insulin binding antibodies and no change in insulin dose. Mean blood glucose and glycosylated serum protein fell in both groups during the study period but there was no significant difference between the two groups.