Two monoclonal antibodies, one directed against human Ia antigens and the other reactive to both monocytes and blast cells, were evaluated for prolonging skin graft survival in Rhesus monkeys. Various doses of the antibodies were given intravenously until the skin graft was rejected. Compared with a graft survival of five days for untreated control monkeys, five animals receiving anti-Ia had grafts surviving from 13 to 19 days and six animals treated with antiblast/monocyte antibody had grafts surviving from 15 to 22 days. No adverse side effects were seen with treatment with antiblast/monocyte antibodies. However, the dose of anti-Ia antibody was critical in that four monkeys died of anaphylaxis when injected with amounts exceeding 200 microliters of ascites with a titer of 1:10(5).