A quantitative estimation of the amounts of human la (HLADR) and HLA-ABC antigen on a variety of human tissues was performed. Monoclonal antibodies to species-common determinants of HLA-DR and HLA-ABC antigens were absorbed quantitatively with tissue homogenates and cell suspensions, and reassayed for residual activity in a radioimmunobinding assay. Kidney was found to carry 90% as much HLA-DR and 14% as much HLA-ABC antigen as spleen, while liver contained 19 and 9% as much, respectively. Small amounts of both antigens were found on heart; brain carried very little HLA-ABC and virtually no DR. Neither HLA-ABC nor DR was found on erythrocytes or reticulocytes. Of interest was our finding that a small subpopulation of thymocytes (10%) was HLA-DR positive. Platelets contained approximately 5% of the amount of HLA-ABC as spleen and undetectable quantities of HLA-DR, as expected. Chronic lymphatic leukemia (CLL) cells were found to carry 10% as much HLA-ABC and 33% as much DR antigen as spleen, while the values for bone marrow were 15 and 2%, respectively.