The in vitro proliferative response of murine bone marrow cells and spleen cells to bacterial lipopolysaccharide (LPS) and the effect of anti-Ia serum on the response were studied. The incorporation of [3H]thymidine into cells prepared from bone marrow increased in the presence of LPS, but the addition of anti-Ia serum to the cultures reduced the incorporation. Pretreatment of bone marrow cells with anti-Ia serum and complement did not abolish the ability of the cells to respond to LPS, while the same pretreatment destroyed this ability in spleen cells. These results suggest that cultures of Ia-negative bone marrow cells generate Ia-positive cells during the culture period, and the Ia-positive cells are responsive cells to LPS. The proliferative response of 1- or 2-week-old spleen cells was easily suppressed by anti-Ia serum when compared with that of 4-week-old spleen cells. Furthermore, the responses of spleen cells obtained from gamma-irradiated and syngeneic bone marrow cell-reconstituted mice were prominently suppressed by anti-Ia serum in comparison with that of normal adult spleen cells. These findings suggest that LPS-responsive lymphocytes in the developmental stage are quite sensitive to anti-Ia serum. The effect of anti-Ia serum on the maturation of bone marrow-derived lymphocytes was discussed.