A number of investigators have demonstrated that there exists a relationship between Ag receptors of Ts cells (TCR) and their soluble suppressor factors. With a view to elucidating this relationship, the primary structures of receptors of Ts cells, induced in mice by tolerogenic conjugates of Ag and monomethoxypolyethylene glycol, were characterized in this study. The cDNA encoding the alpha and beta chains of TCR of cloned Ts cells specific for (i) OVA and (ii) human monoclonal (myeloma) IgG (HIgG) were produced by polymerase chain reaction. From the analysis of the V alpha genes of TCR of Ts cells it was deduced that these receptors utilized a new member of the V alpha 15 gene family, which was productively joined to the J alpha genes that differed for each of the Ts cells of the two distinct specificities. Similarly, sequence analysis of the beta chain cDNA of the two Ts cell clones revealed that both clones utilized the V beta 8.2 gene, and that their J beta gene differed from each other. It is inferred that the Ts cells generated in response to the different tolerogenic Ag(mPEG)n conjugates belonged to a subset of T cells utilizing similar TCR alpha beta chains and differed only in their J alpha/J beta regions. Most importantly, pretreatment of mice with a mixture of pentadecapeptides comprising the TCR alpha chain of the OVA-Ts cells, down-regulated the immune response specific to OVA, but not to HIgG. Moreover, injection of mice with a pentadecapeptide corresponding to the CDR3 region of the TCR alpha chain of either OVA-Ts or HIgG-Ts suppressed specifically the Ab response to OVA or HIgG, respectively. On the basis of all these results, it is concluded that the CDR3 of the TCR-alpha chain of Ts cells plays a pivotal role in the Ts network underlying the specific down-regulation of the immune responses induced by tolerogenic Ag(mPEG)n conjugates.