Human peripheral blood lymphocytes were cultured in vitro with lipopolysaccharide (LPS) for 48 h. After washing, stimulation with a concanavalin A (ConA) or pokeweed mitogen (PWM) resulted in a synergistic blastogenic response that was greater than the sum of the independently stimulated control cultures. Addition of fresh autologous lymphocytes after LPS preculture produced an additional increment of synergy. The nature of the responding and helping effects was determined by coculturing irradiated or nonirradiated lymphocyte suspensions that had been precultured with either LPS or ConA/PWM. Such studies indicated that amplification was the result of a mitogen-activated helper activity, which facilitated the blastogenic response to LPS. Experiments with lymphocytes resolved into T- and B-cell-enriched fractions indicated that the LPS-responsive cells were of the B type and that the help was provided by a mitogen-activated T-cell population. These studies indicated that LPS can induce human B-cell blastogenesis; however, a helper function must be provided by a T-cell subpopulation. This helper activity is inducible by pretreatment of the T cells with plant lectins.