Serum IgA concentrations were found to be normal in a boy with ataxia-telangiectasia and selective IgA deficiency several months after an infusion of bone marrow cells from an HL-A and MLR compatible normal sib. In addition, transient improvements were noted in both his in vivo and in vitro cellular immune responsiveness through the third month posttransplant. Despite this, no evidence of chimerism could be detected in karyotypic studies of his peripheral blood lymphocytes and bone marrow cells. No clinical or significant immunologic improvement was noted in his affected female sib following implantation of a fetal thymus or injection of transfer factor.