This paper describes an L3T4+, Lyt-2- cloned T cell line of CBA/N origin that is specific for IEk-encoded products. Splenic dendritic cells are the predominant stimulatory populations for Lbd cells, being approximately 100 times more effective than large activated B cells. Small B cells and T cells are nonstimulatory. Lbd cells help large B cells to make polyclonal antibody responses, but are unable to activate small B cells and to replace antigen-specific helper T cells in T cell-dependent responses to phosphorylcholine or sheep red blood cells. In the presence of antigen-specific helper T cells, Lbd can amplify, suppress, or contrasuppress T cell function, depending on the type and relative ratios of antigen-reactive cells. We explain our findings as a consequence of the self reactivity among helper T cells.