Activation of natural killer-like T (NKT) cells with the CD1d ligand alpha-galactosylceramide enhances T-dependent humoral immune responses against coadministered T-dependent Ag. At present, there is little information on the mechanisms involved other than a dependence on CD1d expression by antigen-presenting cells and/or development of the NKT subset. We therefore tested the hypothesis that direct presentation of alpha-GC by B cells was required for NKT-enhanced Ab responses against T-dependent Ag. We reconstituted B cell-deficient microMT mice with B cells from C57Bl/6 donors or CD1d(-/-) donors before immunization with NP-KLH alone or NP-KLH mixed with alpha-GC. We made the surprising observation that B-cell expression of CD1d is absolutely required for the NKT-enhanced Ab response. Our data show that the mechanism by which NKT cells enhance humoral immune responses involves interaction with CD1d-expressing B cells.