Interactions between CD40 on B cells and its ligand on activated T cells have been reported to play an important role in T cell-B cell collaboration. In this current study, a mAb against the human CD40 ligand (5c8) was used to investigate the impact of CD40-CD40 ligand interactions in the initial activation of normal human peripheral blood B cells and in subsequent proliferation and differentiation. B cells were activated by co-culture with anti-CD3-stimulated normal T cells. mAb against CD40 ligand blocked initial T cell-dependent B cell activation, as assessed by [3H]uridine incorporation and IL-2R expression. Subsequent B cell proliferation and differentiation were also inhibited by this mAb. In addition to its effect on B cell activation, 5c8 also inhibited the capacity of B cells to augment IL-2 production by anti-CD3 activated T cells, implying a role for CD40-CD40 ligand interactions in the accessory function of B cells. Despite the importance of CD40-CD40 ligand interactions in T cell-B cell collaboration, CD40 ligand-deficient T cell clones were found to induce initial activation of B cells and support Ig production. This effect was only marginally effected by mAb to LFA-1 and ICAM-1, suggesting that additional interaction molecules play a role in T cell-B cell collaboration. Taken together, the data indicate that CD40-CD40 ligand interactions plays an important role in T cell-dependent B cell activation and subsequent differentiation but additional interaction structures are also involved in T cell-B cell collaboration.