Monoclonal antibody (mAb) therapy is a novel alternative treatment for lymphoid malignancies. In this report we present a 55-year-old patient with B-chronic lymphocytic leukemia, who was initially treated with chlorambucil p.o. and subsequently with cyclophosphamide iv with poor response. Then Campath-1H mAb was administered. He received three cycles of Campath-1H, over a 3 yr period, lasting 12 weeks each, at a final dose of 30 mg weekly, on an outpatient basis. After each cycle of Campath-1H administration there was a significant decrease of the size of the palpable lymph nodes, spleen and liver. Restoration of the blood lymphocyte count to normal and a significant decrease of the bone marrow lymphocytic infiltration was observed at the end of each cycle. Therefore, a major clinical response was obtained after all cycles. Campath-1H administration was well tolerated without causing any serious toxicity.