The p55 gag gene of simian immunodeficiency virus macaque strain (SIVmac) and the core p27 gag component linked to a synthetic AUG codon have been cloned into adenovirus type 5 vectors to generate either viable E3-replacement or defective E1-replacement viruses. The viruses express the expected SIV proteins in both human and, for the non-defective viruses, monkey cells. A considerable proportion of the p55 produced is exported from the infected cell. These viruses should prove useful both in studies of the immune response to SIV and as components of candidate vaccines aimed specifically at provoking cytotoxic T cell responses.