It is generally held that the lymphocytes of the thymus are derived from blood-borne cells that migrate into the primordium of the thymus at an early stage of development. These invading precursor cells are thought to originate from mesenchymal elements in the yolk-sac blood islands. In the last few years, I have amassed evidence to question the orthodox doctrine that vertebrate lymphopoietic cells owe their origin to migrant yolk-sac hemocytoblasts. In the leopard frog (Rana pipiens), it appears likely that the lymphocytes of the thymus do not arise from blood-borne stem cells that enter the thymic primordium from the blood islands of the yolk region. Either there is a fundamental difference between the origin of thymic lymphocytes in the amphibians and amniotic vertebrates, or the interpretation in birds and mammals is based on experimental situations that are highly artificial. The role of the thymus will be further elucidated by bringing into play several microsurgical procedures that are ideally suited to amphibian embryos - the parabiotic union of embryos, the induction of triploidy, and thymectomy. We anticipate that there will be a substantial interchange of lymphoid cells between two chromosomally disimilar (diploid and triploid) frog embryos joined in parabiotic union. The extent of cell chimerism in the thymus and peripheral lymphoid tissues (spleen, kidney, and bone marrow) will be ascertained by cytophotometric procedures (relative DNA content of cells). Interest will focus on whether extirpation of the thymic rudiment of one member of the pair will eliminate the cellular chimerism in the thymectomized number. BIBLIOGRAPHIC REFERENCES: Volpe, E. Peter and James B. Turpen. Massive colonization of the spleen by thymus-derived and lymphoid cells. In "Phylogeny of Thymus and Bone Marrow-Bursa Cells" (Ed., R.K. Wright.) Elsevier/North Holland Biomedical Press: Amsterdam, Netherlands. (1976, In Press.) Volpe, E. Peter and James B. Turpen. Central role of the thymus in the immune system of the frog. Transplantation Abstracts (Sixth International Congress of the Transplantation Society): 100, 1976.