Normal lymphocyte maturation is blocked in mutant C.B-17scid mice; consequently, they lack functional T and B cells. However, some scid mice are "leaky" (approximately 15%); i.e., they spontaneously produced large quantities of immunoglobulin (Ig); preliminary evidence suggests that the B cells responsible are few in number and may all derive from a common B lymphocyte stem cell (B-LSC). We propose to test this possibility in several ways using Igh allotypes and novel Igh gene rearrangements as clonal markers. Assuming that monoclonality will apply to most leaky scid mice, we will evaluate the capacity of B-LSC for lymphocyte generation and replacement (persistence and quantity of Igh allotype levels) and diversity (number and kind of Igh-V genes expressed). This capacity will be compared to that of less committed hematopoietic stem cells (HSC). That is, we will deliberately make scid mice "leaky" by reconstitution with B (and T) lymphocyte progency of a single normal HSC. Reconstitution will be accomplished by co-transfer of limiting numbers of BALB/c (Igh-a) and C.AL-20 (Igh-d) fetal liver cells into C.B-17scid (Igh-b) neonates (and embryos). Igh allotypes will serve as clonal markers. Varying the genotype and age of donors and recipients will enable us to examine both T and B cell lineages and the effect of different microenvironments on stem cell growth and differentiation. The value of the scid mouse model for the proposed study is that it enables us to assay the in vivo potential of stem cells at different stages of development and in the context of a normal developmental progression. Many reconstituted scid mice may show unrestricted lymphoid differentiation; i.e.; both T and B lymphocyte replacement. Leaky scid mice, on the other hand, should enable us to study restricted lymphoid differentiation; i.e., B lymphocyte generation in the absence of T cells and in the absence of recruitment of less committed HSC (functional T cells have not yet been detected in any scid mice).