In an 8-year-old boy with acute lymphoblastic leukaemia not previously treated intravenous administration of a single 8 mg dose of a monoclonal antibody that recognizes an epitope restricted to the surface of mature T-cells resulted, within 20 hours, in a fall in circulating lymphoblasts from 200 000 to 70 000. No adverse clinical or biological reaction was detected and no antigenic modulation occurred at lymphoblast surface. This observation constitutes a first step in the complex development of effective serotherapy for malignant diseases, using monoclonal antibodies.