Minor differences in the expression of individual H-2K and H-2D antigens were detected on mouse spleen cells. The method involved the use of an 125I-protein A radioimmunoassay using highly specific anti-H-2 sera to make estimates of the number of cell-bound antibody molecules. The maximum number of antibody binding sites varied for each H-2 antigen reflecting differences of between 10 and 70 percent in the expression of any two antigens. The order of magnitude of expression was Db greater than (Kd)=Kk=Kb=Dq greater than Dd greater than Kq greater than Dk. Minor background differences were detectable, but antigen expression was allele-specific and independent of the expression of other K, D or I antigens expressed on the same cell.