We examined mouse immune response to 4 kinds of recombinant vaccinia viruses carrying the HIV gag gene, including vac-gag/pol, which produces HIV-like particles with processed gag proteins; vac-gag, which also produces HIV-like particles but with unprocessed gag protein; and vac-gag-pol-fuse and vac-es-gag/pol, neither of which produces such particles but releases reverse transcriptase and gag protein, respectively, from infected cells. Although infection of mice with recombinant vaccinia viruses induced production of the anti-p24 antibody in all mice, vac-gag/pol and vac-es-pol induced higher production than the other two recombinants. Increase in [3H]thymidine uptake by splenic lymphocytes following p24 antigen stimulation was most evident in mice infected with vac-gag/pol. Thus, the highest immune reaction, both humoral and cellular, was elicited by vac-gag/pol, indicating that among those tested, this recombinant vaccinia virus is the best candidate for a vaccine that induces anti-HIV gag immunity.