In the continuation of our research on the induction of lymphocytes we will place more emphasis on the elucidation of the biology of the phenomenon and less on the biochemical aspects. Since the discovery of thymopoietin by G. Goldstein (who has since become a member of the team) and of other potent thymic factors, (e.g. the thymosin of A. Goldstein) that produce an abundant source of inducer with a defined chemical composition, the search for further inducers assumes less importance. We will continue to search for precursor cells of thymocytes in yolk sac cells, fetal liver, spleen and in thymus of 14 to 15 day embryos. The aim here is to trace prothymocytes further back in their ontogeny, possibly to the physiological stem cells. We will exploit the two newly described properties of T cells: 1) the potential to react with anti- brain cells serum which has been reported to possess stem cell activity, and 2) the immunoglobulin of T cells. We hope to gain information as to the number of steps necessary from the CFU-S to the thymocyte. So far evidence is lacking that induction of prothymocytes is accompanied by immunological maturation, and we now entertain the hypothesis that additional differentiational steps are required until functions such as GVH, and help in B-T cooperation are attained. Thus, we will test the ability of subpopulations of thymic cells to convert to helper cells on contact with thymic hormones or endotoxin.