AbstractLymphocyte precursors in bursa of Fabricius, thymus and bone marrow (BM) of chick embryos were studied at different stages of incubation over 12–21 days, and for their state of commitment to B or T cell lines of development. Cell suspensions were fractionated on albumin gradients to remove nonlymphoid cells and incubatedin vitrowith bursopoietin, a specific inducer of B cells, or crude chicken thymus extract, a specific inducer of T cells, or ubiquitin, a nonspecific inducer. Precursors were identified by increases in numbers of cells bearing surface alloantigens as determined by immunofluorescence, either Bu‐1 (specific to B cells) or Th‐1 (specific to T cells). Precursors inducible to Bu‐1+cells were found in bursal cells and BM cells from all age groups but not in thymic cells. Precursors inducible to Th‐1+cells were found in thymic preparations and BM cells at all ages, but in significant numbers in bursa on day 12 only. Because B and T precursors were never found together in bursa or thymus, or only in very unequal amounts, it was concluded that precursors in these organs were not multipotential but were separately committed to one or other line of development. This argument did not apply to BM cells, for which other evidence was obtained. Bu‐1+cells were specifically induced in BM cells with bursopoietin and then removed by complement‐dependent cytolysis with anti‐Bu‐1 antiserum. When the remaining cells were incubated with ubiquitin, only Th‐1+cells were induced, showing that Bu‐1 and Th‐1 precursors were separately committed. Surface IgM was never induced on either bursal or BM lymphocytes. The Ia (or B‐L) antigen was inducible on 12‐ to 21‐day bursal cells, but could not be generated on BM cells until day 14 onwards. The pattern of occurrence of committed lymphocyte precursors in the developing chick embryo suggests that these cells are released into the circulation from both central lymphoid organs at their respective times of high lymphopoietic activity, and accumulate in the BM at least up to the time of hatching. Moreover, the presence of committed B precursors in bursa and committed T precursors in thymus at times and in quantities appropriate to the known features of avian lymphopoiesis leads us to conclude thatin vitroinduction is analogous to a true stage ofin viv