Vaccinia virus (VV) infection induces protective T- and B-cell responses, making recombinants based on VV good candidates for the development of effective vaccines to other viruses. VV recombinants expressing the human immunodeficiency virus (HIV) envelope protein (Env) have been generated in several laboratories and shown to induce anti-HIV cellular and humoral immune responses in vaccinated humans and in chimpanzees. To increase the immunogenicity of the Env antigen, a VV recombinant was generated that expresses a chimeric antigen consisting of the Env protein fused to an immunostimulatory cytokine, granulocyte-macrophage colony-stimulating factor (GM-CSF). The chimeric protein retained GM-CSF biological activity when expressed by this recombinant virus (VV-GM-gp120) in cells infected in vitro. Infection of BALB/c mice with VV-GM-gp120 triggered a higher HIV-specific cellular immune response, as measured by interferon-gamma production, than that induced by a VV recombinant expressing the native Env protein. Moreover, although anti-gp120 antibody titres were similar in sera from mice inoculated with either of the VV recombinants, immunization with the recombinant expressing the fusion protein elicited antibodies against a broader spectrum of Env epitopes. These results indicate that HIV Env antigen fusion to GM-CSF provides a means to improve the anti-HIV immune response.