A strain of spontaneously hypertensive rats (SHR) had a selective depression of T-cell functions associated with an early appearance of natural thymocytotoxic autoantibody and a deficiency of thymic hormone. In this paper, the ability of immunopotentiators (IPs) to restore immune functions and to induce antitumor resistance in the T-cell depressed SHR is investigated. In addition, the effect of IPs on activities of natural killer (NK) cells and macrophages is also studied. The results indicate that administration of SPG or Lysozyme enhanced T-cell functions as detected by the rosette formation test and blastogenic responses to PHA. None of the IPs used promoted NK cell activity but Lysozyme and PS-K rather suppressed. In contrast, all IPs used had a effect on activation of macrophages and especially PS-K showed strong activating effect. Following survival and tumor rejection in SHR transplanted with a syngeneic tumor after treatment with the IPs, treatment with SPG produced significant prolongation of survival days but the remaining two IPs had no effect. We propose that the SHR is a suitable animal for quantitative evaluation of the IPs in inducing T-cell mediated immunity an antitumor immune responses.