The cellular enzyme-linked immunospecific assay (CELISA) quantitates the binding of antibody to cell-surface antigens. When conditions that produced the maximum specific enzymatic activity of the conjugate were used, the CELISA was able to screen hybridoma supernatant fluids for polymorphic anti-HLA monoclonal antibodies using as few as 20,000 normal human peripheral blood mononuclear target cells per well. The CELISA detected as little as 24 pg (0.55 fM or 330 X 10(6) molecules) of cell-surface HLA alloantigen and as little as 40 pg of monoclonal antibody bound to cell-surface antigen. In conjunction with various monoclonal antibodies to human peripheral blood mononuclear cell differentiation antigens, the CELISA was used to quantitate levels of different peripheral blood mononuclear cell subsets in normal and diseased people objectively, thus providing a simple, sensitive, and inexpensive alternative to immunofluorescence microscopic and flow cytometric techniques normally used to phenotype peripheral blood mononuclear cells.