Treatment of a 5-month-old male with sex-linked lymphopenic immunological deficiency utilising immunologically competent cells from peripheral blood buffy coat and bone-marrow of a sibling donor resulted in reconstitution of both cellular and humoral immunity. Fatal graft-versus-host disease was circumvented by using donor cells which were histo compatible with the patient's cells with respect to the HL-A locus, as determined by both mixed lymphocyte cultures and lymphocytotoxic assay. A mild graft-versus-host reaction appeared at 8 days post-implantation but resolved spontaneously. Biopsies of rectal mucosa and skin indicate a continuing round-cell infiltration of host tissue 2 months post-implantation; the patient, however, remains clinically well.