C3H/HeN mice were immunized by skin painting with trinitrochlorobenzene or by inoculation of fluorescein isothiocyanate (FITC)-conjugated syngeneic cells. Approximately four weeks later, spleen cells were removed, irradiated, and cocultured with spleen cells from normal mice. These cultures were stimulated with syngeneic cells conjugated with different concentrations of trinitrobenzene sulfonate (TNP-self) or FITC (FITC-self). The results demonstrate (a) appreciable helper cell activity from TNP-self and FITC-self-primed mice, as determined by enhanced hapten-specific cytotoxic T lymphocyte (CTL) responses, (b) that FITC helper activity enhanced the FITC-self CTL more efficiently than the TNP-self CTL response, and vice versa, (c) an alloantigenic stimulating population failed to activate TNP-self helper activity and (d) that the helper activity was not exclusively specific, since FITC helpers did enhance the anti TNP CTL response, and TNP helpers could enhance the anti-FITC-self CTL response. The results of this study indicate that these cell-mediated lympholysis models permit an analysis of hapten specificity and cross-reactivity at the different levels of cellular interaction involved in the generation of an H-2-restricted CTL response.