We have developed a quantitative competitive PCR (QC-PCR) assay in a microplate format for quantifying human immunodeficiency virus Type 1 (HIV-1) DNA or RNA in a broad range of source materials. Our QC-PCR assay is a modification of technique originally described by Piatak et al. (1993), which is based on the presence of a competitive internal standard containing an internal 80-bp deletion of HIV-1 gag target sequence. For improved detection and quantification of the wild-type and internal-standard PCR products in a microplate format, we introduced a non-HIV, 31-bp insert into the internal standard as a probe hybridization site that does not cross-hybridize with wild-type HIV-1 products. By using a primer pair in which one primer is biotinylated, QC-PCRs can be bound to a streptavidin-coated microplate, denatured and probed with a digoxigenin (Dig)-labeled, wild-type or internal-standard probe. The hybridized Dig-labeled probes are detected with an anti-Dig antibody conjugated to detector molecules for luminometry (aequorin) or optical densitometry (peroxidase), yielding results that are quantifiable over the range of 100-10,000 copies of HIV gag. Tested source materials for HIV-1 DNA or RNA quantification include plasma, vaginal lavage and cultured cells. The application of the QC-PCR assay using the microplate format affords a convenient and cost-effective method for quantifying HIV-1 proviral and viral loads from a variety of body fluids, cells and tissues.