Previous studies demonstrated strain-specific differences in opsonic requirements and virulence in neonatal rats among organisms of the same serotype of group B streptococci. The present study investigated the role of sialic acid cell-surface determinants in differences in virulence. Serotype III strains resistant to opsonization by antibody-containing sera had a higher percentage of cellular sialic acid than other organisms (1.02% sialic acid for four type III resistant strains versus 0.59% for three type III sensitive strains; P less than 0.01). Neuraminidase removed up to 70% of the extractable sialic acid from serotype III strains, markedly decreased their virulence, and made them more sensitive to the effects of protective antibody. No differences in sialic acid content were detected between sensitive and resistant strains of serotype II, and neuraminidase did not significantly remove sialic acid or affect their virulence. The present studies suggest differences in the nature or mode of attachment of virulence factors between types II and III of group B streptococci.