We evaluated the survival of highly purified freshly isolated pancreatic islets transplanted from single canine donors into 20 outbred mongrel dogs immunosuppressed with cyclosporine or untreated. The grafts (mean weight +/- SE, 0.5 +/- 0.1 g, containing 122 +/- 8 X 10(3) islets; purity 91% by electron microscopy) were transplanted into 3 groups of dogs: group 1, autograft without CsA (5444 +/- 688 islets/kg body weight, n = 6); group 2, allograft without CsA (6669 +/- 1744, n = 4); and group 3, allograft with CsA (8645 +/- 1149, n = 10). The CsA was injected i.m. daily for 4 days before and 30 days after transplantation. Fasting plasma glucose (PG, mg/dl) and serum CsA trough values were determined daily. Intravenous glucose tolerance tests were done before and after transplantation, for calculation of K values (decline in glucose, %/min; preoperatively, mean K = 3.9 +/- 0.2). In group 1 all 6 dogs were normoglycemic (PG = 98 +/- 2 and K = 1.8 +/- 0.2) at 1 month; in group 2 the graft failed in all 4 dogs, at 4 +/- 1.2 days; in group 3 all 10 dogs were normoglycemic initially. Of the group 3 dogs, 4 died (intussusception developed in 2, and the graft failed at 3 and 9 days in 2 the CsA values of which were less than 300 micrograms/L preoperatively), but the other 6 were still normoglycemic when the CsA was stopped at 30 days (mean PG = 132 +/- 16 and K = 0.9 +/- 0.2; P less than 0.05 vs. group 1). Their CsA values were 708 +/- 197 before and 359 +/- 41 micrograms/L during the third week after transplantation; their grafts failed 12.3 +/- 3.4 days after the cessation of CsA. This data is unique in demonstrating prolonged function of purified allogeneic islets transplanted from individual outbred canine donors, but glucose tolerance was impaired. CsA at serum levels greater than 300 micrograms/L induced prolonged survival of purified canine islets and rejection was prompt when it was stopped.