T10B9.1A-31 is an immunoglobulin Mk (lgMk), CD3b class, murine monoclonal antibody. It is not itself mitogenic, but it partially blocks mitogenesis produced by concanavilin-A, phytohemagglutinin, and a soluble antigen cocktail and is lytic for human peripheral blood T lymphocytes (PB-T) in the presence of fresh autologous human complement and rabbit complement. It reacts with a monomorphic epitope found on all mature PB-Ts that modulates in vitro and in vivo with the CD3/T-cell antigen receptor (TCR) complex, but unlike OKT3, which reacts with CD3 proteins, T10B9.1A-31 reacts with an epitope of the TCR alpha/beta heterodimer. Immunoprecipitation of HPB-ALL with T10B9.1A-31 or OKT3 revealed that T10B9.1A-31 does not react with the 20-to 26-kd polypeptides that compose CD3, and T10B9.1A-31 failed to bind to the PEER cell, which is CD3 gamma/delta positive, TCR alpha/beta negative. Phase 1 clinical trials demonstrated that T10B9.1A-31 caused a rapid decrease in PB-Ts and no toxic effects other than fever, chills, rigors, and transient hypotension. These side effects were absent in methylprednisolone (MP)-pretreated patients. Phase II studies revealed that T10B9.1A-31 reversed steroid refractory acute rejection in three of five renal allografts treated with cyclosporine (CyA) and prednisone (P) maintenance immunosuppression, reversed steroid and polyclonal antisera refractory rejection in three cardiac allograft recipients receiving CyA/P, and attenuated acute rejection in three of three renal patients receiving baseline azathioprine/prednisone (AZA/P) when used as primary therapy. Rerejection was not seen when patients received CyA baseline immunosuppression but was seen in patients maintained on AZA/P OKT3 was efficacious in treating rejection following T10B9.1A-31 therapy. Side effects of T10B9.1A-31 appears to reverse acute rejection crisis effectively in renal and cardiac transplantation with a low incidence of rerejection in CyA-maintained patients and with fewer, milder side effects. Sequential therapy with OKT3 is possible because OKT3 and T10B9.1A-31 are of different isotypes and idiotypes.