Induction of H-Y-specific cytotoxic T lymphocyte (CTL) responses in nonresponder female mice was attempted by i.v. injection of allogeneic male cells, followed by in vitro restimulation of recipient spleen cells with syngeneic male cells. Responses were obtained only in two strain combinations in which the recipients, although phenotypically nonresponders, carried responder alleles at class I major histocompatibility complex (MHC) loci, and the immunizing cells differed from the recipients at class II MHC loci. The two positive strain combinations were B10.A(2R) anti-B10.A(4R), and B10.GD anti-B10.D2(R101). In the first combination, both recipient and donor are nonresponders to H-Y, and the CTL are induced via a bystander effect of another CTL response to a previously undetected minor histocompatibility (H) antigen. This "carrier" antigen can only induce CTL against H-Y and itself when the immunizing cells express class II MHC molecules. Furthermore, the presence of H-Y and the carrier antigen on the same cell is a prerequisite for the generation of H-Y-specific CTL. In the second combination, the recipient is a nonresponder, whereas the donor is a responder. The two strains differ at only E alpha and E beta class II MHC loci. For the induction of CTL, H-Y and the foreign E molecule must be expressed on the same cells. Thus, the B10.D2(R101) cells that express E molecules on their surface probably provide the E-nonexpressor B10.GD recipients with a stimulus for the generation of H-Y-specific T helper cells. The data are consistent with the notion that antigen-specific class II MHC-restricted T helper cells are involved in the initiation of CTL responses to minor H antigens.