Reagents and assays for detecting HIV-1 groups M and O and optionally HIV-1 group N and SIVcpz are provided. The reagents are nucleic acid primers for the hybridization to, amplification and subsequent detection of HIV-1 groups M, N and O and SIVcpz in a biological sample. The primers are oligonucleotides that selectively hybridize to the highly conserved regions of the env and pol regions of HIV-1. Due to the high sensitivity of the assays, small concentrations of HIV in a biological sample can be detected, allowing diagnosis at an early stage of infection. The assays are qualitative or quantitative and are useful for viral load determinations of HIV-1 groups M, N or O in a patient undergoing treatment for HIV-1 infection. Viral load determinations can be used to monitor the progress of the treatment regimen, the development of drug resistance, and to predict disease progression.