A clear-cut serological differentiation between AKR lymphocytes of thymic and non-thymic origin is reported: these two cell types are antigenically distinct. In newborn mice, the AKR thymic antigen was found at a high concentration only in thymus. In adult mice, the antigen was present at a high level in thymus, all nervous tissues tested, and some leukemias. It was present at much lower levels in lymph node lymphocytes, splenic lymphocytes, appendix, lung, and certain other leukemias, which appeared to be of non-thymic origin. The AKR thymic antigen was present at a high level in thymus and nervous tissues of RF mice, but was absent from thymocytes of sixteen other mouse strains. These sixteen strains possessed the C3HeB/Fe thymic antigen. The distribution of this antigen in neonatal and adult tissues of the strains tested was similar to that of the AKR thymic antigen in AKR mice. No exceptions were found. These results were obtained by use of immune cytolysis, and of a new method for the quantitative treatment of data from absorption experiments.