Replication competent chimeric viruses that express the gag and pol proteins of SIVmac and the env proteins of HIV-1 were made. One such chimeric virus, SHIV-4, that expresses the vif, vpx, vpr, and nef regulatory genes of SIV and the tat and rev regulatory genes of HIV-1 replicated efficiently in cynomolgus monkeys. This model system can be used to evaluate the efficacy of anti-HIV-1 vaccines directed at the envelope glycoproteins, anti-HIV-1 envelope glycoprotein antiserum or monoclonal antibodies, and anti-HIV-1 drugs designed to inhibit the tat, rev, or env functions.