Lymphoid cell engraftment was monitored for several years after bone marrow transplantation by Y-chromatin staining of T and B lymphocytes in the peripheral blood and/or by immunoglobulin allotyping in the serum of 20 of 52 pediatric patients grafted successively between October 1973 and October 1983. Data on 2 patients with severe combined immunodeficiency, grafted earlier in December 1968 and April 1971, are also included. These children received an allogeneic bone marrow graft for leukemia (n = 7), severe aplastic anemia (n = 11), or severe combined immunodeficiency (n = 4) and were informative for this study, because they differed from their donor by sex (n = 16) and/or by immunoglobulin phenotype (n = 13). Of 16 pairs in which the donor was of the opposite sex, 11 patients ultimately showed circulating T and B lymphocytes of donor origin after bone marrow transplantation; in the remaining 5, there was an incomplete chimerism of the circulating lymphoid cells. Of 13 pairs with a difference in immunoglobulin phenotype between donor and recipient, 8 patients exhibited donor allotypes 3 months or later after transplantation, in 3 of them together with recipient allotypes. In the remaining 5 patients, recipient allotypes were detected after transplantation, but the simultaneous presence of donor-type immunoglobulin production could not be excluded in 4. The persistence of either a split (T lineage of donor origin and B lineage of recipient origin) or mixed (T and/or B lineage of donor and recipient origin) chimerism was related to the type of disease. In 3 children circulating B cells of donor-origin did not fit with the recipient origin of the sessile immunoglobulin-secreting plasma cells. This implies that different immune compartments--e.g., bone marrow and peripheral lymphoid tissues--should be investigated following allogeneic bone marrow transplantation. A prolonged presence of recipient-type lymphoid cells increased the risk of leukemic relapse in the patients investigated.