Our studies reported here, fully characterize two unique type 2 antigens trinitrophenol (TNP)-M1 serotype carbohydrates (TNP-M1 g and TNP-M1 c) derived from streptococci, which fail to induce antibody responses in xid or neonatal mouse splenic cultures. These antigens generate brisk responses in normal spleen and in Peyer's patch cell cultures of xid mice, all of which suggest that responses are elicited in the Lyb-3 + , 5 + B subpopulation. The antibody responses to TNP-M1 g (and TNP-M1 c) are not dependent upon T cells. Furthermore, TNP-M1 carbohydrates induce anti-TNP plaque-forming (PFC) responses in cultures of small, resting splenic B cell populations without an added T cell requirement. Thus two categories of type 2 antigens are distinguished, one which requires T cells or derived factors, e.g., TNP-Ficoll, and a second TNP-carbohydrate antigen TNP-M1 that does not. Studies of the mitogenic and polyconal B cell activation properties of M1 carbohydrates indicated that B cell proliferation is induced in both xid (Lyb-3 - , 5 - ) and normal (Lyb-3 - , 5 - and Lyb-3 + , 5 + ) splenic B cell subpopulations, but that differentiation to IgM synthesis fails to occur in the Lyb 3 - , 5 - B cell subpopulation. Thus M1 carbohydrates are unique probes that allow the selective induction of proliferation and differentiation of mature B cells that are presumably Lyb-3 + , 5 + . Because the M1 serotype carbohydrates induce polyclonal IgM synthesis and antigen-specific responses in only the mature B cell population in the absence of T cells, whereas TNP-Ficoll and other type 2 antigens require T cells or their derived factors, the Lyb-3 + , 5 + B cell subpopulation may consist of a T cell-dependent and a T cell-independent compartment for responses to different carbohydrate type 2 antigens.