The involvement of HLA‐class I in target cell lysis by CD4⁺ cytolytic T cells (CTL) has been a controversial issue. A CTL clone of CD4 phenotype was derived from the peripheral blood lymphocytes of a patient with primary melanoma. The CTL clone stably lysed the autologous primary melanoma cells for approximately 9 months in culture. Both the Vα2/Vβ8 T‐cell receptor and CD4 were involved in CTL cytotoxicity. Of a large panel of allogeneic primary and metastatic melanoma or colorectal carcinoma cells, autologous and allogeneic Epstein‐Barr virus‐transformed B cells and autologous fibroblasts, only allogeneic metastatic melanoma cells matched with the autologous tumor cells for HLA‐class I (B57[17]) were lysed and induced IFN‐γ secretion by the CTL clone. Lysis of the autologous tumor cells was significantly blocked by monoclonal antibody to HLA‐B17. Importantly, allogeneic, HLA‐class I‐ and class II‐unmatched melanoma cells were lysed by the CTL only following transfection of the cells with B57[17] cDNA. Our results provide direct evidence for the involvement of both CD4 and HLA‐class I in tumor cell lysis by CD4⁺ CTL. Int. J. Cancer 85:253–259, 2000. ©2000 Wiley‐Liss, Inc.