The tissue distribution of the canine and human homologues of Thy-1 were studied using quantitative absorption analyses of liver absorbed anti-brain xenosera assayed on thymus cells. Cross-reactivity studies with pure rat Thy-1 established that the assays were detecting the homologues of rat Thy-1. The results showed that the tissue distribution of Thy-1 varies remarkably between species. Canine Thy-1 was found in large amounts only on adult brain. It was present in much smaller amounts on thymus (8% compared with brain), and was also found on lymph node, spleen and bone marrow (3, 1 and 0.5%, respectively, compared with brain). Surprisingly, it was found on kidney in amounts equal to that on thymus. Studies with the fluorescence-activated cell sorter established that canine Thy-1 was present on all thymocytes and peripheral T lymphocytes. Neonatal thymus and kidney had adult levels of Thy-1, but only small amounts of Thy-1 were present on neonatal brain. In man, brain was again the only tissue to contain large amounts of Thy-1. Surprisingly, it was absent from human thymus, and present on human kidney in amounts roughly equivalent to that seen in the dog. It was absent from spleen, lymph node, bone marrow, liver, heart, erythrocytes, platelets, and serum. Cross-reactivity studies showed that dog and human Thy-1 showed extensive serological cross-reaction, and that the dog-rat and human-rat cross-reactive components were identical.