Lymphoid fetal liver cultures (LFLC) are long-term, nontransformed cultures of early B lymphoid lineage cells which appear developmentally blocked at the pre-B stage in vitro. When injected into severe combined immunodeficient (SCID) mice, cells from LFLC could reconstitute splenic B lymphocytes and serum IgM. T lymphocyte reconstitution was not observed and serum IgG levels were very low. IgG3 was the predominant gamma subisotype in the serum of the LFLC-reconstituted mice, indicating impaired class switching in these B lymphocytes. When thymocytes were coinjected with LFLC, the B lymphocytes were able to class switch fully and respond to T-dependent antigens. These serological responses were heterogeneous. This experimental system allows separation of three B lymphocyte developmental stages: early differentiation in vitro, progression to IgM secretion in vivo, and late differentiation dependent upon mature T lymphocytes in vivo. The unique advantage of this system is the ability to regulate the B lymphocyte developmental pathway in a defined, stepwise manner.