A serogroup 29E Neisseria meningitidis capsular polysaccharide vaccine at a dose of 50 micrograms was injected subcutaneously into 10 adult human volunteers. One vaccinate experienced a mild systemic reaction; one complained of moderate to severe local pain and tenderness. The vaccine induced significant homologous binding and bactericidal antibody by 2 weeks and significant binding antibody against the heterologous serogroup Z capsular polysaccharide by 4 weeks. Although binding antibody rose during the first 4 weeks and then declined slowly over the subsequent 4 months, bactericidal antibody response declined substantially by 4 and 8 weeks for both polysaccharides. The increase in group 29E bactericidal activity was no longer significant at 4 and 8 weeks; loss of bactericidal activity against group Z was significant by 8 weeks. Bactericidal activity again rose between 8 and 26 weeks, becoming significantly increased over prevaccination levels for group 29E and increased, though not significantly (P = 0.085), over prevaccination levels for group Z. As a result of the failure of induced antibody to increase bactericidal activity, only 40% of vaccinates achieved a greater than 2 log2 increase in lytic activity against group 29E, and none achieved this large an increase in lytic activity against group Z.