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A hemagglutinin extracted from the L-phase of Neisseria meningitidis group B directly agglutinated rabbit and rooster erythrocytes but did not react with human, sheep, or guinea pig erythrocytes. Virtually all animal, chicken, and human sera tested showed inhibitory titers. In a significant number of Navy recruits, who became carriers of N. meningitidis, the titer of serum inhibitor began to decline 2 weeks before the bacterial infection was first detected. Titers of this serum inhibitor ranged from 256 to 4,096 in patients with meningitis. The hemagglutinin failed to boost the preexisting titers of serum inhibitor when injected into animals, although roosters showed a minimal but transitory response.