Antibody-dependent activity against group C meningococci mediated by human mononuclear cells or purified lymphocytes was inhibited when sera from adults immunized with group C meningococcal polysaccharide were preincubated with that polysaccharide. Likewise, activity against group A meningococci induced by heat-inactivated sera from adults immunized with group A meningococcal polysaccharide was inhibited by preincubation with that polysaccharide. Neither heterologous polysaccharide nor homologous protein or lipopolysaccharide meningococcal antigens inhibited the activity of either of these sera. The data indicate that immune specificity in this cell-mediated antibacterial system is dependent upon antibodies to the specific polysaccharide antigen with which the serum donor had been immunized.