The earliest T-cell precursor population in the adult mouse thymus (low CD4 precursors) may be divided into 85% of cells expressing surface Mel-14 (LECAM-1, the lymphocyte homing receptor) and 15% of cells which are Mel-14. To date, this is the only surface marker for which we have found this population to be heterogeneous. The precursor activity of the Mel-14+ and Mel-14- subpopulation was assessed by both intrathymic and intravenous transfer of sorted cells into Ly5 congenic irradiated recipient mice. On both a cell-for-cell and a total activity basis, almost all precursor activity was associated with the Mel-14+ cells. No segregation was seen between T-cell, B-cell and dendritic cell precursor activity of the low CD4 population, all activities being concentrated in the Mel-14+ fraction. This strengthens the hypothesis that one precursor cell has the potential to form all three lineages.