It is now established that monoclonal antibodies (MAb) against LFA-1 and Lyt-2,3 antigens on cytolytic T lymphocytes (CTL) block killing function in the absence of C. It has been suggested that the blocking is inversely related to CTL-target affinity. In this report, we studied the effect of papain pretreatment of target cells, because papain is known to remove H-2 and to render target cells more resistant to allospecific CTL. CTL-target conjugate formation was weaker with papain-treated target cells (based on reduced post-dispersion lysis in dextran-containing medium). The concentration of MAb required to produce 40 to 60% inhibition of 51Cr release (2-hr assay) was reduced four to 29-fold for alpha LFA-1 and 64 to 114-fold for alpha Lyt-2,3. Papain, however, did not induce blocking by MAb to other CTL antigens such as Thy-1, H-2, and T200. Flow cytometric analysis confirmed that papain selectively removed more than 95% of H-2. In kinetic studies of removal and recovery, H-2 density and conjugate formation correlated well with each other. Sensitivity to blocking was not as well correlated, raising the possibility that an unidentified papain-sensitive target cell molecule other than H-2 plays an important role in CTL-target interaction.