The humoral immune response of conventionally raised BALB/c mice to the so-called "thymus independent" antigen alpha (1- > 3) Dextran B 1355 S (Dex) is predominantly of the IgM class. The response is further characterized by Igha allotype linkage and the dominance of the public idiotypes (Id) J558 and MOPC 104. In germfree raised BALB/c and in BALB/c nu/nu mice immunized with the same antigen an additional IgG response of the public Id is observed. Analysis of the regulation of the class expression reveals existence of specific Ts cells in euthymic mice which must have been activated pre- or perinatally by exposure to environmental bacterial antigens. They permit or enforce differentiation of Dex-specific B cells into B gamma memory cells without allowing further development into IgG producing plasma cells. An analogue of these splenic Ts cells has now been cloned and identified as an I-Ed restricted Id-specific T cell with exactly the properties ascribed above to the splenic Ts cells. This paper describes phenotypical and functional properties of the Ts cell clone 178-4. It evaluates this clone's role in controlling efficient anti-bacterial IgM-mediated immunity under conditions where a class switch to IgG antibody production is actively suppressed; possibly as a measure to avoid hazardous autoimmune reactions on the basis of crossreaction and antigenic mimicry between polysaccharide antigens.