An in vitro system for transforming immature lymphoid cells present in the thymus at early development has been established. By phenotype analysis of the transformants obtained, we observed that B cell precursors, susceptible to Abelson murine leukemia virus (A-MuLV)- or Harvey murine sarcoma virus (H-MuSV)-induced lymphogenesis, were present at high frequency in the fetal thymus of BALB/c mice. These precursors recolonized alymphoid thymus lobes in vitro, as do T cell precursors. It was further observed that B precursors in the fetal liver were also capable of recolonizing alymphoid thymus lobes and were stored in a thymic environment. These results suggest that stroma cells of the fetal thymus may possess the capacity to support the growth of B precursors. On the other hand, B cell precursors sensitive to the viral transformation were undetectable in the fetal thymus of C57BL/6, although immunohistochemical analysis suggested their presence. However, in the fetal liver of the same strain, B precursors recolonizing alymphoid thymus in vitro were sensitive to the viral transformation. Based on these results, we will discuss both the role and fate of thymic B precursors. In addition, we also obtained T cell lymphomas at different stages of differentiation from the fetal thymus of C57BL/6 infected with A-MuLV or H-MuSV. These data indicate the usefulness of our system in establishing cell lines derived from intrathymic lymphogenesis at early development.