During ontogeny, T cell precursors must colonize the thymus to acquire immunocompetency. Using migration assays, a chemotactic activity was detected in conditioned media from avian embryonic thymic epithelial cells. The responding cells were shown to acquire T lymphocyte markers after homing into the thymus. Absorption experiments demonstrated surface receptors for the chemotactic substance on these hemopoietic precursors, which were not found on thymus-derived lymphocytes. Two peaks of chemotactic activity in the 1 kd-4 kd molecular weight range were detected after fractionation of thymic epithelial cell-conditioned medium. One of these activities was retained after heating to 95 degrees C but was destroyed after proteolytic treatment. Thus chemotactic peptides may be responsible for the thymic recruitment of the first hemopoietic precursors and may also be involved in the renewal of these precursors throughout adult life.