The nature of primed precursor T cells (primed pre-TD), capable of differentiating into effector T cells (TD) that mediate delayed-type hypersensitivity (DTH), was investigated in B10 mice which were primed by intravenous (iv) injection of various doses of sheep red blood cells (SRBC). The presence of primed pre-TD was detected by the ability of T cells in the spleens from primed mice, which were treated in vitro with pertussis toxin and then transferred into naive recipient mice, to generate DTH in the recipient mice 14 days after transfer. The primed pre-TD were induced antigen specifically 1 day after mice were primed by iv injection of a suboptimal (10(3)), an optimal (10(5)), or supraoptimal (10(9)) dose of SRBC. They were replaced by TD 4 days after priming in optimally sensitized mice, while they were maintained without generating TD for at least 5 weeks after priming in mice primed with either a suboptimal or a supraoptimal dose of SRBC. They were L3T4-positive and dense cells, fractionated in the high-density layers on a discontinuous Percoll density gradient, and capable of transforming into less dense TD, fractionated in the low-density layers. These results indicate that primed pre-TD, which are induced by an antigen signal and then can be activated by a nonspecific stimulus, are present not only in responsive mice but also in unresponsive mice, suggesting that either the generation of TD from primed pre-TD or primed pre-TD alone is the decisive factor for either responsiveness or unresponsiveness.