Polysaccharide vaccines prepared from group B Neisseria meningitidis were tested in human volunteers. Antibody responses were tested by five different serologic assays. Titration of one vaccine (10-, 50-, 100-, and 250-µg doses) showed only one confirmed rise in antibody to the polysaccharide in 51 men tested. Another 50 volunteers who had received vaccines of different molecular sizes failed to develop significant antibody responses. Eight volunteers who received the polysaccharide of the largest molecular size showed no antibody response; three of these men also failed to respond to a booster injection. Only one individual of four who received a combined influenza virus-meningococcal group B vaccine developed significant increase in titer of antibody to meningococcal polysaccharide. Failure of the group B polysaccharide to stimulate antibody in all but a few individuals may be due to an intrinsic deficiency of the vaccines or may represent an example of immunologic tolerance.