By using a hemagglutination-inhibition assay, certain lots of purified group A meningococcal polysaccharide vaccines were found to contain trace amounts of group C polysaccharide (0.032 to 0.128%). Young adult human volunteers inoculated with these vaccines showed a very poor group C serum anti-polysaccharide antibody response as determined by a radioactive antigen-binding assay. Two weeks later volunteers were given a 50-mug subcutaneous injection of purified group C polysaccharide. Serum specimens obtained 1 month later showed a significant suppression of group C antibody response in comparison with subjects who had initially received group A vaccines which were free of group C antigen. The observed hyporesponsiveness may represent an example of immunologic tolerance in man.