Serological analysis of idiotypic specificities present in azobenzenearsonate (ABA)-specific first-order suppressor T cell factors (TsF1) from C.AL-20 and BALB/c mice revealed a significant difference between TsF from these two strains of mice. The idiotypic composition of TsF1 from BALB/c mice appears to be more heterogeneous, and at least two different fractions can be readily identified. One bears the characteristic BALB/c-associated CRI(C) (crossreactive idiotype) determinants, and the other is non-CRI(C)-bearing. Analysis of ABA-specific TsF1 from animals lacking B cells uncovered a fundamental change in the expression of their idiotypic specificities. TsF from rabbit anti-mouse IgM (anti-mu)-treated C.AL-20 mice failed to express the characteristic CRI(A) determinants. Instead, they express CRI(C) specificities. Similarly, TsF1 from anti-mu-treated BALB/c mice did not express their characteristic CRI(C) specificities, but rather express CRI(A) determinants. These experiments provide strong evidence that the Igh restriction specificity of TsF is dictated by the particular idiotypic specificities expressed. They also clearly demonstrate that B cells and their products play an important role in establishing the idiotypic composition and repertoire of suppressor T cells.