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 pediatrie 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).