A quantitative method for the evaluation of human HLA-DR antigen expression has been developed. Cell membrane proteins were solubilized in Nonidet P-40 or deoxycholic acid detergent and diluted in a Triton X-100 containing sample buffer. The samples were subsequently spotted on a nitrocellulose membrane filter and fixed by immersion in isopropyl alcohol-acetic acid solution. The membrane was saturated in a 5% BSA blocking buffer and sequentially incubated with specific monoclonal anti-HLA-DR antibody, and 125I-labelled protein A. Each spot was then assayed for radioactivity in a gamma scintillation counter. Immunoadsorbant purified HLA-DR antigen was used to standardize the method and a reference dosage curve was established with serial dilutions of the purified HLA-DR antigen. The method permitted the detection of HLA-DR antigens with reproducibility in the ng range, in cellular extracts, physiological and pathological fluids, and in fractions eluted from affinity columns.