Anti-CD4 monoclonal antibodies (mAbs) have been shown to inhibit in vitro T-cell activation and proliferation to both antigens and mitogens. Animals studies have demonstrated the immunosuppressive potency of anti-CD4 mAbs given in vivo for therapy of autoimmune disease and following allografting. Similarly, leflunomide (LF), a new potent immunosuppressive, has been shown to be effective in preventing autoimmune disorders and reactions leading to organ transplant rejection. LF is thought to antagonize cytokine activity, thereby interfering with T-helper-cell-dependent B- and T-lymphocyte proliferation. A new anti-CD4 antibody (RIB 5/2) was investigated alone and in combination with cyclosporin A (CsA) and LF for the treatment of corneal allograft rejection in the rat. Corneal buttons were grafted from Lewis/Brown Norway rats to Lewis recipients. Animals were randomly assigned to the following treatment groups: (1) untreated; (2) CsA at 1.5 mg/kg; (3) RIB 5/2 at 2.5 mg/kg; (4) RIB 5/2 at 2.5 mg/kg and CsA at 1.5 mg/kg; (5) RIB 5/2 at 2.5 mg/kg, CsA at 1.5 mg/kg and LF at 10 mg/kg; (6) RIB 5/2 at 4 mg/kg; (7) RIB 5/2 at 4 mg/kg and CsA at 1.5 mg/kg; and (8) RIB 5/2 at 4 mg/kg, CsA at 1.5 mg/kg, and LF at 10 mg/kg. RIB 5/2 was given intraperitoneally at 24 h before surgery, on the day of grafting, and on postoperative day 1 and was continued every 2nd day until the rat had received ten doses in all (postoperative day 15).(ABSTRACT TRUNCATED AT 250 WORDS)