A limiting-dilution system is described that makes use of T cell growth factor T cell expansion and allows the determination of precursor frequencies for various regulatory and effector T cells in nonimmune, polyclonally, or specifically activated T cell populations. Two different sets, a frequent and a rare set, of T helper cell precursors with specificity for trinitrophenyl-group A streptococcal vaccine, could be identified: the frequent set is of the Lyt-123 phenotype, and is present at frequencies of from 1/1,000 to 1/6,000 splenic T cells. It is only active at low cell numbers, whereas it is completely inactivated at greater cell numbers, presumably by suppressor T cells of lower frequency but greater potency. The rare set is of the Lyt-1 phenotype, is present at frequencies of from 1/10,000 to 1/70,000, and is not sensitive to suppressor cells present within the tested cell numbers. We suggest that the frequent set contains primiary helper cell precursors, whereas the rare set contains helper T memory cells preselected by previous exposure to other antigens. The results are discussed with respect to other reports on the involvement of more than one set of helper cells in antibody production.