The clinical effects of a murine monoclonal anti-human T-cell antibody were studied in 7 patients with T-cell lymphoma. Four to 17 treatments with anti-Leu-1 were given to each patient over periods of 14-75 days. Doses of antibody ranged from 250 .mu.g to 100 mg. Antibody treatments usually caused a rapid fall in circulating T cells, with return to baseline levels within 24-48 h. The optimum dose appeared to vary for each patient. Clearance of circulating tumor cells correlated with the amount of antibody bound to cells. Other than dyspnea in 1 patient, no serious toxicity was noted. Five patients had definite tumor responses, but these were of short duration (1.5-4 mo.). Four patients developed anti-mouse immunoglobulin (Ig) antibodies, and in 3 patients, this was responsible for tumor escape from therapy. Although 95% of the host anti-mouse Ig response was directed against mouse Ig constant region determinants, a small but significant component was found to be antiidiotype.