Cytotoxic T lymphocyte (CTL) responses were obtained in vitro to cells coupled with several different haptens. The degree of lysis of target cells was dependent on the amount of hapten coupled to stimulator and target cells. Spleen cells from normal mice responded to high-hapten density cells but not to low-hapten density cells. However, spleen cells from immunized CBA mice could be stimulated in vitro by low-hapten density cells to generate effector cells able to lyse low-hapten density cells. In vitro primed responder cells could be restimulated in the presence of the original hapten-coupled stimulator cells or in the presence of supernatant from concanavalin A-stimulated mouse or rat spleen cells. Large number of hapten-specific and H-2-restricted CTL could be generated by repeated exposure to fresh supernatant.