The lysis of murine sarcoma virus-murine leukaemia virus (MSV-MuLV)-induced tumour cells by cytotoxic T lymphocytes (CTL) appears to require that an antigen specified by MSV-MuLV, or induced by the infection, be presented in association with class I major histocompatibility complex antigens 1,2. The viral proteins of the tumorigenic MuLV seem to be a part of the antigens recognized by these dually restricted anti-MuLV CTL, but the precise nature of the putative viral antigen(s) recognized by CTL is unknown. Studies using recombinant viruses have suggested that a product of the viral envelope gene (env gene), perhaps the glycoprotein gp70, is the viral antigen recognized by CTL3. Attempts to use purified gp70 or anti-gp70 antibodies to block CTL recognition of retrovirus-induced tumour cells, however, have yielded contradictory results4-6. To examine more closely the role of gp70 in the CTL response to MuLV infections, we have constructed murine cell lines which express only the env gene of the Moloney murine leukaemia virus (M-MuLV). We show here that BALB/c-3T3 cells expressing the M-MuLV envelope gene products on their cell surface are susceptible to lysis by M-MuLV-specific CTL.