The tissue distribution of major histocompatibility complex (MHC) antigens in the DA and Lewis (LEW) strains was studied using LEW anti-DA and DA anti-LEW alloantisera. Quantitative absorption analyses were used with quantitative binding assays for SD and Ia antigens. Initial screening showed that the LEW anti-DA serum contained significant amounts of antibodies against both Ia and SD antigens. On the other hand, the DA anti-LEW serum seemed to be directed almost entirely against Ia antigens, and it was not possible to set up assays for SD antigens in the LEW strain. The most surprising finding was the presence of large amounts of Ia antigen on the kidneys of both the DA and LEW strains, one kidney containing as much Ia antigen as half a spleen. Kidney also contained large amounts of SD antigen. Liver had large amounts of SD, but very little Ia. Heart had only small amounts of both SD and Ia. The relevance of these findings to transplantation of the kidney, liver, and heart are discussed. The other tissues studied were brain, spleen, lymph node, thoracic duct lymphocytes, bone marrow, thymus, RBC, and platelets. The most interesting findings were the presence of relatively large amounts of SD antigens on DA RBC, and small amounts of Ia on the thymus and bone marrow.