We report here that an interleukin-3-dependent precursor B-cell line, LyD9, differentiated in vitro into mature B cells, producing immunoglobulin (Ig)M and IgG by co-culture with bone marrow stroma cells. Induced LyD9 cells underwent heterogenous immunoglobulin gene rearrangement and synthesized mRNAs encoding immunoglobulin mu (mu), gamma (gamma) and kappa (kappa) chains. LyD9 was also shown to differentiate into myeloid cells. We have established an interleukin-4-dependent derivative clone K-4 that is an intermediate between myeloid-lymphoid cells and the LyD9 clone. This differentiation required direct contact between LyD9 and stromal cells.