A method for quantitative determination of mouse Thy‐1 antigen is presented. Antigen to be assayed is incubated with anti‐Thy‐1 serum and remaining antibody activity is measured by adding51Cr‐labelled thymocytes and complement. The detection limit corresponds to the amount of Thy‐1 antigen exposed on the surface of 25,000 thymocytes, which can be calculated to correspond to approximately 0.5 ng. The assay was designed to be performed in the presence or absence of detergent, and is therefore a useful tool for quantitation of Thy‐1 in purification procedures and studies on the biological significance of this membr