Mice homozygous for an autosomal recessive scid (severe combined immune deficiency) mutation on chromosome 16 exhibit a defect that specifically impairs lymphoid differentiation but not myelopoiesis. Consequently such mice are deficient in both humoral and cell-mediated immune functions. Despite their defect, scid mice survive under pathogen-free conditions and are fertile. The mutation does not impair the hematopoietic microenvironment necessary for lymphoid differentiation, since these mice can be cured with grafts of normal bone marrow (BM) or cells from long-term BM cultures (LTBMC); however, reconstitution requires sublethal (400 cGy) irradiation of recipients. Engraftment with cells from LTBMC gave near-normal levels of colony- forming B cells (CFU-B) in spleen and BM of the recipients by 6 weeks postgrafting. Since LTBMC are devoid of all mature B and pre-B cells but contain stem cells that restore lymphoid function in scid mice, we used a limiting-dilution assay to characterize and enumerate the number of stem cells in LTBMC capable of restoring lymphoid function. Curing was determined by the CFU-B-cell assay, since CFU-B are not detectable in normal scid mice. The results indicate that fewer cells from LTBMC than from fresh BM are required to obtain lymphoid reconstitution. As few as 10(3) LTBMC cells can repopulate significant B- and T-cell function in scid recipients. From these results we conclude that scid mice can be used as recipients to quantify lymphoid-restricted stem cells and that there is a functional separation of lymphoid- and myeloid-restricted stem cells in LTBMC with an enrichment for lymphoid- restricted stem cells in these cultures.