The phenotype of the donor thymocytes present in the thymus of irradiated mice injected intravenously with CD3-depleted total bone marrow cells has been studied by three-color flow cytometry during the time period 6-16 days postinjection. Donor cells could first be reliably detected at Day 6 after reconstitution. Donor and host cells maintain their relative proportions over the first few days, after which time the proportion of donor cells derived from the bone marrow increases. At Days 6 and 7 after reconstitution, Pgp-1+, IL-2R- cells predominate, although a minority of Pgp-1+, IL-2R+ cells is also seen. Few cells are Pgp-1-, IL-2R+. Over the next 3 days, the relative proportion of Pgp-1+, IL-2R- cells declines rapidly and the relative proportion of Pgp-1+, IL-2R+ cells and then of Pgp-1-, IL-2R+ cells peaks and declines. The absolute number of all three populations, however, increases exponentially until Day 14. Most donor cells present at Days 6-7 after reconstitution are L3T4-, Lyt-2-. Over the next 2 days, the majority of donor-derived cells express low, but significant, levels of both L3T4 and Lyt-2. L3T4+, Lyt-2+ cells expressing levels of cell surface antigen characteristic of the cognate population found in the adult first appear at Day 10 after reconstitution. These L3T4+, Lyt-2+ donor cells increase in proportion to reach 70-80% of donor-derived cells after Day 14 of reconstitution. The host thymocyte population, on the other hand, contains few Pgp-1+ or IL-2R+ cells even at 7 days after irradiation and is predominantly L3T4+, Lyt-2+ by Day 8. This observation suggests that the host cells are derived from a more mature precursor than the bone marrow prothymocyte or the earliest intrathymic progenitors.