The studies of the biological effects of OKT3 monoclonal antibody on mature human T cells demonstrate that binding to a differentiation antigen by an antibody can modulate the specialized functions of the cells bearing the antigen. OKT3 induces T cells to proliferate and to secrete gamma interferon. The antibody also inhibits cytotoxic T cells from lysing target cells. The mechanisms involved and the nature of the antigen to which OKT3 binds are discussed.