Cytolytic T cells were generated in vitro by culturing purified Balb/c CD4+ T cells with irradiated C57B1/6 (B6) splenocytes plus anti-IL-4 mAb. Matched, noncytotoxic T cells were similarly generated by culturing purified Balb/c CD4+ T cells with irradiated B6 splenocytes plus recombinant murine IL-4. The latter T cells displayed to cytolytic activity, even in lectin-mediated lysis assays, but produced characteristic cytokines upon contact with specific alloantigens. Transfusion of cytolytic T cell populations into Balb/c SCID mice bearing B6 cardiac allografts resulted in acute allograft rejection within 5 to 10 days. Transfusion of noncytolytic T cell populations into Balb/c SCID mice bearing B6 cardiac allografts also resulted in acute allograft rejection within 7 to 10 days. Limiting dilution analysis (LDA) of infiltrating cells recovered from rejected allografts after collagenase digestion demonstrated that the CD4+ T cells retained their cytolytic or noncytolytic functional phenotypes in vivo throughout the rejection process. These data demonstrate that isolated CD4+ T cell populations can promote rapid acute cardiac allograft rejection, and that cytolytic activity is not necessary for this acute rejection response.