Formation of all blood cells initiates with common, multipotential hemopoietic stem cells. These are thought to give rise to committed precursors which through a series of events expand and differentiate to become functional B lymphocytes. Our studies have been aimed at resolving intermediate cell types in this pathway and probing mechanisms which may be important for their regulation. Methods were developed for obtaining stem cell populations relatively free of their progeny cells and for using functional and chromosomal markers to follow their fate after transfer to irradiated and immunodeficient recipient mice. Cell surface markers and depletion techniques were used to distinguish different types of B cell precursors and determine the time required experimentally for them to give rise to funcional B cells. Monoclonal antibodies were prepared to B lineage antigens and used to enrich for B cell precursors from human and murine bone marrow and from tissues of embryonic mice. These have been studied in detail and compared in terms of size, morphology, ultrastructure, synthesis of immunoglobulin chains, and differentiative capability in vivo and in vitro. Additional studies in progress consider their origins, migration after isolation and reinfusion, and configuration of genes for light and heavy immunoglobulin chains. Proposed research includes plans for determining intrinsic differences in various B cell precursors and ordering these in a differentiation sequence. Culture conditions have been established for observing at least the final stages in B cell formation and emphasis will be placed on understanding how important cellular interactions which take place in those circumstances are mediated. This will include a comprehensive series of studies directed toward identifying macromolecules which affect B cell precursor proliferation and maturation. In addition to these experiments with normal embryonic, neonatal, adult and aging animals, parallel investigations are being conducted with established cell lines which are similar to murine pre-B cells and an earlier type of cells which remain multiple options.