In this report, we describe an immune mechanism that seems to play a role in tumor immunity in BALB/c mice challenged with SV40-transformed cells. This immune mechanism was induced by immunization of mice with rSV40 large tumor Ag (T-Ag). After rSV40 T-Ag immunization, BALB/c mice were protected from a lethal tumor challenge with syngeneic SV40-transformed cells (mKSA). Specifically, we examined CTL activity, NK activity, complement-dependent cytotoxicity, and Ab-dependent cell-mediated cytotoxicity (ADCC) in rSV40 T-Ag-immunized mice that were protected from a subsequent lethal tumor challenge. Immune splenocytes from surviving animals that were subsequently primed in vitro with rSV40 T-Ag demonstrated little to no SV40 T-Ag-specific CTL activity. In addition, natural immunity, as assessed by NK lytic activity, was comparable to that of unimmunized animals. Similarly, minimal significant complement-dependent cytotoxicity of radiolabeled mKSA cells was demonstrated by SV40 T-Ag-specific serum Abs. However, SV40 T-Ag-specific Abs using peritoneal exudate cells as effectors, exhibited significant levels of ADCC against radiolabeled mKSA target cells. Together, these studies indicate that rSV40 T-Ag immunization elicited ADCC that may represent a potential humoral immune mechanism involved in the protection of BALB/c mice from a lethal challenge with syngeneic SV40-transformed cells.