The cellular interactions involved in maintaining CD4+ T-cell memory have hitherto not been identified. In this report, we have investigated the role played by B cells in this process. We show that that long-lasting helper T-cell memory depends on the presence of B cells, but that direct antigen presentation by B cells is not required. These findings provide new insights into the mechanisms which underlie helper T-cell memory. They also suggest that the efficacy of future vaccines will depend critically on the inclusion of B- as well as T-cell epitopes.