A quantitative PCR assay for the detection of HIV-1 nucleic acids is described. The assay is based on a competitive internal standard nucleic acid which can be discriminated from target sequences by the presence of a new restriction enzyme site. The method was used to quantitate plasmid molecules containing HIV-1 sequences, HIV-1 DNA and HIV-1 RNA purified from HIV-1-infected tissue culture cells as well as HIV-1 DNA present in the peripheral blood mononuclear cells of an AIDS patient. The assay will be valuable for assessing viral load in AIDS patients and for the study of viral gene expression.