The present study examines the role of nonclass I MHC target membrane structures involved in lysis by cytotoxic lymphocytes in the lectin-dependent cellular cytotoxicity (LDCC) system. Cytotoxic cells of rodents and humans have been shown to nonspecifically lyse target cells of different origins and species. The mechanism by which such cytotoxicity takes place is not known, although several hypotheses have been put forth. This study examines the role of class I MHC expression on target cells and Lyt-2 antigens on the cytotoxic cells in both allogeneic and xenogeneic LDCC systems. Human peripheral blood lymphocytes, and murine-allosensitized cytotoxic T lymphocytes (CTL) are used as effector cells, and class I MHC-negative Daudi and class I MHC-positive Raji human target cells are used. The studies reveal that in LDCC, human lymphocytes lyse HLA-negative Daudi targets to the same extent as HLA-positive Raji targets. This was shown by the 51Cr-release assay as well as the single cell assay. Furthermore, allosensitized murine CTL lyse Daudi and Raji in LDCC, demonstrating that lysis of class I negative targets is achieved by xenogeneic CTL. Target cells treated with papain were found to be resistant to lysis in LDCC. However, incubation of papain-treated targets for 1 to 2 hr resulted in recovery of cytotoxicity. In the single cell assay, the papain treatment of targets slightly reduces the frequency of binders but significantly reduces the frequency of killers, revealing that the papain-sensitive structures are essential during the lethal hit stage of lysis. Monoclonal anti-Lyt-2 antibody inhibits lysis of both Daudi and Raji targets by murine CTL. The lack of discrimination between mouse and human targets by murine CTL suggests that the Lyt-2 molecule may have a different role than merely interacting with the target cell. In the LDCC system, we propose that class I MHC molecules are not essential for lysis. Several papain-sensitive and -insensitive target cell structures may participate in the cytolytic mechanism, and we propose that both the lectin and Lyt-2 molecules are involved in the perturbation of cell membrane conformation essential for lysis.