T/B lineage choice in migrant thymic progenitors occurs prior to thymus entry and contact with Notch ligands on thymic epithelium. CD45⁺ cells from E12 fetal liver, perithymic mesenchyme, and thymic epithelium introduced into alymphoid 2-dGuo–treated thymus lobes have the ability to give rise to differentiated T-cell progeny defined by CD4 and CD8 (A) including cells of both αβ and γδ T lineages (B-C). The B-cell potential of E12 precursors from these sources was also compared by culturing them on monolayers of OP9 bone marrow stromal cells in multiwell plates. Results are presented as the CD19⁺ cells within the CD45⁺ fraction of cells recovered from each well (D). Vertical lines in the histograms represent background staining controls. Average total numbers of CD45⁺CD19⁺ cells recovered per well for each precursor input population are shown in panel E. ▦ indicates E12 fetal liver; □, E12 perithymic mesenchyme; and ▪, thymic epithelium. Note the lack of B-cell development from thymus-derived precursors. Each experiment was performed 4 times with similar results. Data are expressed as an average ± SEM.