The effect of (alpha 2-->8)-linked polysialic acid on the adherence of Neisseria meningitidis to human mucosal cells was examined using a serogroup B-encapsulated strain and a capsule-defective (Cap-) mutant of this strain. The Cap- mutant contains a single truncated insert of Tn916 in a 3.8-kb HaeIII chromosomal fragment. The Tn916 insert was shown to be responsible for the phenotype by linkage studies and by demonstration that loss of the insert restored encapsulation. The Cap- mutant consistently adhered to human buccal epithelial cell in greater numbers than the encapsulated parent, but the increase in adherence was less than twofold. Adherence of the Cap- mutant during infection of human nasopharyngeal organ cultures was 1.3- to 6.5-fold greater than that of the encapsulated parent. However, specificity of adherence of meningococci for nonciliated nasopharyngeal epithelial cells and the ability to be internalized by these cells was not due to the (alpha 2-->8)-linked polysialic acid capsule.