A murine hybridoma monoclonal anti-T-cell antibody was given to a patient with advanced cutaneous T cell lymphoma. The patient tolerated seventeen treatment courses over ten weeks without symptoms of toxicity. Each treatment caused a reduction in circulating T cells which reached a stable level after two weeks of therapy. Antigenic modulation was observed after each treatment but was fully reversed within three to four days; therefore the treatment was given twice weekly. Monoclonal antibody produced a striking clinical response in skin, lymph nodes, and blood but complete remission was not achieved. Since the remaining disease was antigen positive, failure to eradicate all tumour cells was not a result of immunoselection. At no time did the patient show an antimouse immune response.