Assays of H-Y-specific cell-mediated cytolysis (CMC) in vitro were carried out with B6 female effector cells and B6 male target cells. This lytic system depends on compatibility for H-2Db but not for H-2Kb. Antibody to H-2Db but not to H-2Kb blocked the CMC assay. However, maximal blocking required exposure to the target cells to H-2Db antibody for up to 2 h. This interval is far longer than would be required for blocking of a pre-existing H-Y:H-2Db complex, and seems more consonant with a membrane reorganization bringing H-Y and H-2Db together, as proposed from mapping studies. The question whether the postulated configurational change affecting H-Y and H-2Db is a necessary feature of CMC awaits further study.