An indirect radioimmunoassay was used to detect Ia antigenic determinants at the cell surface of thymic and splenic cells. An A.TH anti-A.TL serum was used as a source of anti-Ia antibodies which could bind to H-2a or H-2k target cells. Bound alloantibody was then detected with 125I anti-mouse IgG1 antibody. This assay system revealed Ia antigens on thymic T cells which are not usually detectable by cytotoxicity assays, and has the potential for quantitative measurement of such determinants on cells. Critical adsorption studies were performed to confirm that Ia antigens were in fact the targets of the alloantiserum tested.