Chemically modified lipopolysaccharides of Salmonella abortus-equi were tested for mitogenicity on mouse spleen cells as well as antagonism of the mitogenicity of intact lipopolysaccharide (LPS). All the lipopolysaccharide preparations deacylated by different alkaline treatments suffered a drastic loss of mitogenicity. The mitogenic activity of lipid A was also lost when succinic residues were introduced on hydroxyl groups. Partially deacylated alkaline-treated preparations (but not completely deacylated preparations) inhibited the activation of splenic B-cells by LPS. They were found to be toxic to spleen cells, however, and to suppress not only the mitogenicity of LPS but that of concanavalin A as well. This inhibitory action was not exhibited when all of the fatty acid was eliminated. Succinylated lipid A, on the other hand, was not toxic to the cells and inhibited the B-cell mitogenicity of lipopolysaccharide (but not the T-cell mitogenicity of concanavalin A). Chemical analysis revealed that about 4.6 mol of succinic acid had been introduced into lipid A by succinylation, and that the fatty acid and phosphate composition was unchanged by this treatment. Macrophages do not seem to participate in this inhibition. Inhibition was observed when succinylated lipid A was added either at the same time or after lipid A mitogen, but optimal inhibition was expressed when it was added to the culture 3 h before LPS. Inhibition was not affected by washing the cells before adding LPS. Inhibition increased as the ratio of suppressor to mitogen increased, suggesting that the succinylated lipid A competes with intact LPS.