Antigens of Streptococcus mutans 6715 (alternatively designated serotype g Streptococcussobrinus), including whole cells (WC g), cell walls (CW g), peptidoglycan (PG g) and serotype carbohydrate (Ml g) were coupled with trinitrophenyl (TNP), and the nature of the immune response to each immunogen was determined in normal and X-linked immunodeficient (xid) murine spleen cell cultures. Responses to TNP-WC g, -CW g and -PG g and to the classical type 1 antigen TNP-Brucella abortus occurred in both xid and normal splenic cultures, while TNP-Ml g only triggered immune responses in normal spleen cell cultures, suggesting that the former three antigens are type 1 and the latter type 2. Further support for the type 2 nature of TNP-Ml g was the finding that Peyer's patch cell cultures from both xid (which contain mature B cells) and normal mice supported responses to TNP-Ml g and TNP-Ficoll, while xid splenic cultures failed to support responses to either type 2 antigen. The three type 1 TNP-S. mutans antigens induced responses in nude spleen cell and in purified splenic B cell cultures, but required T cells for in vitro responses to lower doses of immunogen. On the other hand, TNP-Ml g induced anti-TNP PFC responses at several antigen concentrations in purified B cell cultures, without requirement for added T cells. These studies show that the intact S. mutans cell, as well as CW g and PG g, acts as a T cell-dependent (TD) type 1 antigen, while the serotype carbohydrate (Ml g) induces a T cell-independent (TI) type 2 response. Thus, the intact bacterium is a TD type 1 antigen, whereas its purified components are either type 1 or type 2 antigens and differ significantly in terms of their T cell dependence. © 1987, Gustav Fischer Verlag · Stuttgart · New York. All rights reserved.