Further advances are required in understanding protection from AIDS by T-cell immunity. We analyzed a set of multigenic simian/human immunodeficiency virus (SHIV) DNA and fowlpox virus priming and boosting vaccines for immunogenicity and protective efficacy in outbred pigtail macaques. The number of vaccinations required, the effect of DNA vaccination alone, and the effect of cytokine (gamma interferon) coexpression by the fowlpox virus boost was also studied. A coordinated induction of high levels of broadly reactive CD4 and CD8 T-cell immune responses was induced by sequential DNA and fowlpox virus vaccination. The immunogenicity of regimens utilizing fowlpox virus coexpressing gamma interferon, a single DNA priming vaccination, or DNA vaccines alone was inferior. Significant control of a virulent SHIV challenge was observed despite a loss of SHIV-specific proliferating T cells. The outcome of challenge with virulent SHIV(mn229) correlated with vaccine immunogenicity except that DNA vaccination alone primed for protection almost as effectively as the DNA/fowlpox virus regimen despite negligible immunogenicity by standard assays. These studies suggest that priming of immunity with DNA and fowlpox virus vaccines could delay AIDS in humans.