Donor and host T cells were distinguished by T cell antigen marker Thy-1 isotype and cytoplasmic isozyme Gpi-1 in this study of bone marrow transplantation between congenic mice. During the first 3-6 weeks after irradiation and marrow transfer, percentages of cells bearing the donor Thy-1 isotype in the periphery are much lower than percentages of T cells bearing the donor Gpi-1 marker. Apparently a population of Thy-1- donor T cells exists for several weeks after bone marrow transplantation. Further study showed that this population of CD3+, Thy-1- donor T cells expressed CD4+ or CD8+ and was found in peripheral blood and spleen but not in the thymus. This finding suggests their extrathymic origin.