An antigen-specific subcellular component that augments anti-hapten IgE class homocytotropic antibody (HTA) response was extracted from thymocytes and spleen cells of rats hyperimmunized with dinitrophenylated Ascaris suum extract (DNP-As) or with Ascaris protein (As) alone. This soluble component, when injected into neonatally thymectomized rats, restored the ability to produce anti-DNP HTA and hemagglutinating antibodies of the recipients that were otherwise incapable of producing antibodies upon immunization with DNP-As. The thymocyte extract from either normal rats or those hyperimmunized with DNP-bovine serum albumin (DNP-BSA) did not show the restoring ability for both HTA and hemagglutinating antibodies. Furthermore, this “helper” activity was largely absorbed by the carrier protein, As, but not with hapten, DNP-BSA, indicating the helper component possesses specificity and affinity for the carrier molecule. Although these antigen-specific characteristics of the helper component are much like those of the previously reported “inhibitory” component of thymus-derived lymphocytes that specifically suppresses the ongoing HTA response, a number of differences in their immunochemical and physicochemical properties were observed. Unlike the inhibitory component, the helper component possessed Fab and µ-chain determinants as revealed by absorption studies. The molecular weight of the helper component was between 200,000 and 100,000 daltons, the value much larger than that obtained for the inhibitory component. These results indicate that IgT-like molecule is involved in the induction of in vivo HTA response, and further suggest that HTA formation is regulated by two distinct antigen-specific subcellular components of thymus-derived lymphocytes in the rat.