Abstract: Diluent and wash compositions are useful in a rapid and sensitive assay for detecting antibodies, and especially retroviral antibodies, in a biological specimen. The diluent composition is buffered to a pH of 6 to 10 and includes a protein or carbohydrate, a surfactant and a negatively-charged organic compound. The wash composition is buffered to a pH of 5 to 10 and includes a surfactant. These compositions can be included in a diagnostic kit. The method of this invention includes mixing the biological specimen with the diluent composition, forming an immunological complex between ligand and antibodies in the specimen and separating complexed materials from uncomplexed materials using a filtration membrane and a washing step. An enzyme labeled anti-antibody is added to form a ligand-antibody-antibody complex followed by its detection using suitable reagents.