A T-cell hybridoma clone, which produces antigen-specific helper factors and a T-cell lymphoma clone which produces non-specific helper factors was used to study the expression of T-cell allotypes and Ia antigens. Use was made of rabbit antisera against isolated T-cell receptor material and of monoclonal mouse antibodies against isolated rat Ia antigen. The rabbit antisera detected endogenously produced determinants both on the membrane and on intracellular polypeptides of these cells. The monoclonal mouse anti-rat-Ia antibodies detected polymorphic determinants on mouse Ia antigens and reacted with endogenously produced molecules on the membrane and on intracellular molecules of the hybridoma and lymphoma cells. The molecules carrying Tcr allotypes were single-chain polypeptides with mol. wts of 60,000-70,000 and the molecules carrying Ia-like antigenic determinants were single-chain polypeptides with mol. wts of 40,000-50,000. Thus T-cell allotypes and Ia antigens were found on separate polypeptide chains. The role and genetic localization of allotype-like and Ia-like molecules in T-cell products is discussed.