We report the results of a prospective non-randomized phase II study of Muromonab-CD3 (Orthoclone OKT3 (R)), an anti-CD3 monoclonal antibody, with methylprednisolone (MP) and cyclosporine (CSA) for acute GVHD (aGVHD) prophylaxis in 22 hematologic malignancy patients. OKT3 was given at 0.1 mg/kg/day with a maximum dose of 5 mg/day. Initial MP dose was 1000 mg before OKT3, with subsequent doses at 1 mg/kg/day before each OKT3 infusion with a planned taper beginning at day +28. CSA (3 mg/kg/day) was given as a continuous infusion at day -1 and adjusted to maintain serum levels between 250 and 399 ng/ml. Allogeneic BMT donors were HLA-matched siblings (n -17), single HLA-mismatched-related (n 1) and HLA-matched unrelated (n 4). All patients achieved neutrophil engraftment at a median 11 days (range, 8 -25 days). By intent-to-treat, the cumulative incidence of grade II -IV aGVHD was 33% (95% CI 13 -53%) at a median 26 days post-BMT (range, 14\-84 days). Chronic GVHD developed in 11/12 evaluable patients. Eight patients (36%) developed OKT3 first dose reactions; no cases of post-transplant lympho-proliferative disorder were observed. OKT3 depleted peripheral CD3+ cells in vivo as measured by flow cytometry. OKT3+MP+CSA combination is moderately effective aGVHD prophylaxis, however, it is unlikely to be superior to CSA+MTX.