Bifunctional antibodies specific for a determinant within the T cell receptor (TcR) complex of cytotoxic T lymphocytes (CTL) and a determinant expressed on the surface of the target cell will effectively mediate cytolysis. In such a lytic system anti-Lyt-2 antibody can block cytolysis. We have observed that the amount of inhibition varies considerably from clone to clone and surprisingly correlates well with inhibition of conjugate formation as mediated by bifunctional antibody. This implies that inhibition of antibody-mediated killing occurs as the result of reduction of the avidity of the effector cell for its target, the same mechanism responsible for inhibition of receptor-mediated lysis by anti-Lyt-2. In light of the similarity between the mechanism of inhibition by anti-Lyt-2 of receptor-mediated and antibody-mediated cytolysis, we compared the ability of anti-Lyt-2 to inhibit cytolysis in these two different assay systems by using a number of different CTL clones. Whereas the majority of secondary CTL clones (presumed to have high affinity TcR) are inhibited equally in both assay systems, most primary CTL (presumed to have low affinity TcR) are more susceptible to inhibition by anti-Lyt-2 in their receptor-specific than their antibody-directed cytolysis. These results, taken together with an apparent correlation between the amount of Lyt-2 expressed on the cell surface and susceptibility to inhibition, suggest anti-Lyt-2 may block CTL function by sterically inhibiting mobility of the TcR complex.