Several functional and phenotypic B cell populations have been described in the spleen. These include the 'transitional' subsets, which are thought to be late differentiation intermediates of marrow-derived, mature follicular B cells. The exact progenitor-successor relationships of these transitional subsets, as well as whether a proliferative step is requisite for follicular B cell maturation, remain controversial. Moreover, whether late B cell differentiation might involve branched or asynchronous maturation pathways, thus allowing some cells to 'skip' one or more of these stages, has not been investigated. Herein we have used mathematical modeling to interrogate these possibilities. Using mathematical models that numerically simulate splenic B cell population dynamics, we have determined which alternative models of differentiation best fit existing in vivo labeling data. Our results indicate that follicular differentiation does not involve a proliferating splenic intermediate. Our results further suggest that some developing cells move directly from the immature marrow pool to more advanced semi-mature peripheral subsets without passing through the least mature subset in the spleen.