Inbred, male Lewis rats underwent heterotopic heart allografting from F344 donor rats, or streptozocin (STZ)-induced diabetic Lewis rats underwent pancreas allografting with bladder drainage from F344 or ACI donor rats. A monoclonal antibody (MoAb) to intracellular adhesion molecule 1 (ICAM-1) was given i. p. (1.0 mg/kg) for 10 days, and its immunosuppressive potency was evaluated. The mean survival time (MST) of the heart allografts was significantly prolonged in the MoAb-treated group. Both exocrine and endocrine MST of pancreas allografts were also prolonged by MoAb administration across the minor and major histocompatibility barriers. However, complete graft tolerance was not induced. Our study demonstrated that the MoAb to ICAM-1 alone can delay the allograft rejection in rat organ transplantation.