A monoclonal antibody (H-99) was prepared that reacts specifically with the leukemia cells from a patient with stage IV chronic lymphocytic leukemia of B-cell origin. Escalating doses of the antibody were administered to the patient in two courses of therapy. The first course did not result in any significant change in total lymphocyte count; however, the platelet count (which was initially abnormal) rose steadily during treatment to a level greater than 250,000 per mm3. A second course of treatment was begun approximately three months after the first course, and the protocol was modified to consist of five doses of antibody given over a 10-day period, with the highest dose being 500 mg per day. The second course of treatment did not result in any change in total lymphocyte or platelet levels, and there was no significant toxicity associated with the treatment. The failure to induce an anti-tumor response in the patient was probably related to the large concentration (approximately 400 micrograms/mL) of free serum idiotype in addition to the severely compromised immune system of the patient at the time of treatment.