Whether T cell tolerance represents direct inactivation of antigen-specific T cells via recognition of antigen plus major histocompatibility complex, or via T suppressor (Ts) cells, or a combination of these mechanisms, remains to be clarified. This problem was investigated using a novel approach based on the finding in several systems that T helper/proliferative (Th/Tp) cell-inducing antigenic determinants are dissociable from Ts cell-inducing determinants. Thus, peptide probes containing known sites that stimulate T proliferative activity, as well as peptides from distinct sites assumed to bear Ts-inducing determinants, were used in studying hen (chicken) eggwhite lysozyme (HEL)-tolerant mice. The clear prediction from clonal deletion model is that Th/Tp response potential to short peptides in the tolerant mouse would not exist, while regulatory suppression models predict the coexistence of antigen-reactive cells and antigen-specific regulatory cells that prevent their expression. Adult mice, treated with 2 mg HEL in saline, were tolerant to HEL in complete Freund's adjuvant (CFA). Latent T cell proliferative responses could be revealed to determinants within two HEL peptide probes, which lacked the amino-terminal region of the molecule. This responsiveness suggested two conclusions: first, Ts cells directed against the amino terminus of lysozyme exist in the tolerant genetic responder B10.A; second, these Ts regulate the activity of functional antigen-reactive T cells directed against epitopes elsewhere on the molecule, but only in the presence of the complete molecule, HEL. Examination of neonatally induced tolerance did not reveal any latent responsiveness, supporting the hypothesis that clonal deletion or anergy is the relevant mechanism in this situation. Possible reservations in these explanations of the two tolerant states, plus analysis of the more complex "split tolerance" resulting from 20 mg HEL in saline treatment in adults, are discussed. The approach of dissociation of proliferation-inducing determinants from suppression-inducing determinants clarifies our understanding of the tolerant state and holds promise for more definitive exploration of mechanisms of T cell tolerance.