An immunoperoxidase method has been developed which allows accurate and sensitive quantitation of the binding of monoclonal antibodies to cell surface antigens. Monolayers of fixed cells were prepared in wells of Terasaki micro-test plates and monoclonal antibodies bound to cell surface antigens were identified by the unlabeled antibody-enzyme method of Sternberger (1974). The cell-bound peroxidase could either be quantified per well or visualized on individual cells by the use of appropriate substrates for peroxidase. Experimental procedures are described in detail and results obtained with several monoclonal antibodies with specificity for different target cells are shown. Limitations and applications of the technique are discussed.