PURPOSE: Severe combined immunodeficiency (SCID) is a syndrome of diverse genetic cause characterized by profound deficiencies of T, B and sometimes NK cell function. Non-ablative HLA-identical or rigorously T cell-depleted haploidentical parental bone marrow transplantation (BMT) results in thymus-dependent genetically donor T cell development in the recipients, leading to a high rate of long-term survival. However, the development of B cell function has been more problematic. We report here results of analyses of B cell function in 125 SCID recipients prior to and long-term after non-ablative BMT, according to their molecular type. METHODS: Studies included blood immunoglobulin measurements; antibody titers to standard vaccines, blood group antigens and bacteriophage Φ X 174; flow cytometry to examine for markers of immaturity, memory, switched memory B cells and BAFF receptor expression; B cell chimerism; B cell spectratyping; and B cell proliferation. RESULTS: The results showed that B cell chimerism was not required for normal B cell function in IL7Rα-Def, ADA-Def and CD3-Def SCIDs. In X-linked-SCID, Jak3-Def SCID and those with V-D-J recombination defects, donor B cell chimerism was necessary for B cell function to develop. CONCLUSION: The most important factor determining whether B cell function develops in SCID T cell chimeras is the underlying molecular defect. In some types, host B cells function normally. In those molecular types where host B cell function did not develop, donor B cell chimerism was necessary to achieve B cell function. 236 words.