When the cells from various lymphoid tissues from normal mice were treated with bacterial endotoxin (LPS) in vitro and transferred to syngeneic mice, the plaque-forming cell (PFC)-response to sheep erythrocytes (SRBC) in the recipients' spleen was markedly enhanced. The enhanced PFC-response of the recipients was attained even after transfer of macrophage-deprived spleen cells. Similar results obtained in heavily x-irradiated recipient mice indicate that the LPS-treated donor cells could account for the enhanced response of the recipients. Enhanced immune response to SRBC in the heavily x-irradiated recipients was also attained when thymus or bone marrow cells were treated with LPS and transferred together with the other. These results suggest that, in its adjuvancy, LPS affects both thymus-derived “antigen reactive cells” and bone marrow-derived “antibody-producing cell precursors,” and facilitates the cell cooperation in the anti-SRBC antibody response.