Mouse splenic B cells can be separated based on their distinctive expression of cell surface antigens. Marginal zone (MZ) B cells are CD38(high) CD21(high) CD23(low/-), while follicular (FO) B cells are CD21(int) CD23(int) and newly formed (NF) B cells are CD21(dim/-) CD23-. Exploiting these phenotypic distinctions, we isolated the three B cell subsets and assessed their other phenotypic differences and functional capabilities in vitro. FO B cells proliferate more than the other B cell subsets in response to either IgM or CD38 cross-linking. MZ B cells proliferate better than FO B cells when stimulated with lipopolysaccharide (LPS), sub-optimal levels of LPS and CD38 cross-linking or CD40 ligation. When NF, FO and MZ B cells were stimulated with either LPS or LPS and interleukin-4, MZ B cells secreted more IgM and IgG3 than the other two subsets. Similarly, calcium fluxes measured within MZ B cells are larger in amplitude and more sustained after B cell receptor cross-linking than those found in the other two subsets. Collectively, these results indicate that CD38(high) CD21(high) CD23(low/-) MZ B cells are specially suited to play a unique role in response to antigens delivered to the MZ area.