Monoclonal antibodies (MCA) specific against one subset of lymphocytes (anti-B) were used in treating skin allograft rejection in Rhesus monkeys. Rhesus monkeys were treated daily with intravenous MCA until the skin graft was rejected. The dose of the MCA was adjusted according to the peripheral B-cell counts obtained by rosetting technique as well as by indirect immunofluorescence. We finally achieved a safe dose, though the margin of safety was narrow. We also noted that the B-cell count dropped to a significant degree and that the survival of the skin graft was prolonged.