The effect of prophylactic i.v. administration of high doses of human γ-immunoglobulin (IgG) on kidney graft survival was investigated in rhesus monkeys treated with azathioprine and prednisolone. In nontransfused recipients not treated with IgG (controls), graft survival ranged from 9 to 22 days; if nontreated animals had been given three pretransplant blood transfusions, graft survival ranged from 9 to 61 days with 42% of the animals showing a prolonged survival time (>22 days). However, in both transfused and nontransfused recipients, the additional pretransplant administration of IgG appeared to have an adverse effect: about 25% of the animals showed accelerated rejection. In addition, serum creatinine levels in IgG-treated recipients were significantly higher on the 3rd day after transplantation than in non-treated monkeys. We concluded that renal transplant patients should be treated with IgG for protection against life-threatening infections only if they have good kidney function.