Syngeneic cells exogenously supplied with hen egg lysozyme (HEL) or endogenously synthesizing HEL were used as antigen-presenting cells to induce major histocompatibility complex class I-restricted cytotoxic T lymphocytes (CTL). Immunization of C57BL/6 mice followed by repeated stimulation of their splenocytes in vitro with trypsinized HEL peptides led to the generation of CTL lines specific for trypsinized HEL peptides and restricted by H-2K. Immunization of C3H mice with a mixture of soluble native HEL and irradiated syngeneic spleen cells followed by in vitro stimulation of immune spleen cells with soluble HEL could in a few cases result in HEL-specific CTL able to kill syngeneic transfectant L cells secreting HEL (HELs) or expressing cytosol-targeted HEL (HELc). The use of HELs or HELc transfectant L cells as in vivo and in vitro immunogens was a potent way for eliciting HEL-specific polyclonal CTL. These CTL and two CD8+ clones were found to be H-2K restricted and specific for the 1-17 N-terminal HEL peptide. In addition, the anti-HEL CTL could also exhibit a significant cross-reactivity against unsensitized and HEL-untransfected targets expressing the K restriction element. This cross-reactivity was likely due to recognition of unidentified HEL mimicking peptides (self-derived?) presented by the MHC class I (H-2K or H-2K) molecule used as the restriction element for the specific recognition of HEL. The CTL raised after immunization with HELs or HELc transfectant cells were found to recognize both the HELs and HELc transfectant cells even though HEL was not detected in the latter after a 2- or 5-min radiolabeling pulse. Recognition of both HELs and HELc transfectant cells by a given CTL clone suggests that HEL subjected to two separate processing pathways, each depending on the initial subcellular localization, can ensure the generation of similar MHC class I peptide complexes.