By rosetting with SRBC coupled to rabbit-anti-human IgM, the surface IgM-negative cells of human fetal bone marrow were enriched, and subsequently infected and transformed by Epstein-Barr virus (EBV). Single clones of the transformed cells were obtained. Ninety percent of the resulting cell clones were surface-immunoglobulin-negative, and of 8 clones which were further studied, 5 lacked intracellular, cytoplasmic Ig as measured by immunofluorescence. Control cell clones derived from the same material without pre-selection expressed surface Ig and also secreted Ig. Utilization of a panel of B-cell-specific monoclonal antibodies (MAbs) showed no difference between the cell clones expressing surface Ig and those that did not. The progenitor B-cell lines did not show a phenotype resembling that of cell lines derived from B-cell malignancies, such as high agarose clonability. In spite of their immature Ig-phenotype, these clones showed rearrangement of at least one heavy chain Ig-allele. Efforts to induce differentiation in these clones were unsuccessful. These clones may represent progenitor B cells, or B cells with faulty heavy-chain rearrangement. EBV can apparently be used as a tool to derive cell lines representing different levels of B-cell differentiation, and can also transform immature B cells, which may be useful in the analysis of B-cell differentiation.