Ig production by peripheral blood lymphocytes of renal allograft recipients was studied. It was demonstrated that lymphocytes of patients with acute graft rejection produce elevated numbers of PFC in culture in response to PWM (a T-cell dependent stimulus) and Salmonella paratyphi B (a T-cell independent stimulus) as well as spontaneously. PFC responses were significantly higher in patients with acute rejection than in a group of patients with stable graft function. The B cells of the recipients with stable graft function showed a normal humoral response in vitro. During chronic rejection no significant changes were found in the production of antibodies by B cells.