The ileal Peyer's patch (PP) plays a central role in B cell development in young sheep and it is hypothesized that this B cell development occurs independent of extrinsic antigen and T cells. Therefore, it was of interest to examine ileal PP folllcular (iPf) B cell responses to CD40 ligand, a molecule integral to T cell-dependent B cell development. A variable level of CD40 expression was detected on a subpopulation of iPfB cells and J558L cells, expressing a membrane form of mouse CD40 ligand (mCD40L), interacted specifically with the CD40 molecule on iPfB cells. In response to mCD40L the non-S phase iPfB cells were rescued from apoptotic cell death and there was a marked proliferative response but viable cell number remained relatively constant. The mCD40L also induced decreased cytoplasmic cAMP levels, blocked anti-Ig-induced iPfB cell death and induced functional IL-2 receptor expression on a subpopulation of iPfB cells. Many of the mCD40L-induced responses of iPfB cells were similar to those reported for germinal centre and immature B cells, and indicated that a cognate T cell-B cell interaction could influence iPfB cell proliferation and differentiation. Finally, that mCD40L induced iPfB cell activation and differentiation was evident as increased expression of CD5, the BAQ44A molecule, the CACT65A molecule and the expansion of surface IgG1+ B cells. These mCD40L-induced phenotypic changes were also observed on subpopulations of freshly isolated iPfB cells and jejunal PP follicular B cells. However, few iPfB cells had a phenotype similar to that observed in co-culture with mCD40L and this suggested that T cell-dependent B cell development may play a minor role in ileal PP B cell development. The possible significance of CD40 signalling is discussed in terms of the selection of iPfB cells during development.