Serum-sensitive Neisseria gonorrhoeae strains become serum resistant when grown in the presence of a sialic acid precursor, cytidine monophospho-N-acetylneuraminic acid. We examined the abilities of human neutrophils to phagocytose sialylated and nonsialylated gonococci and observed a decrease in the complement-dependent phagocytosis of sialylated gonococci compared with that of nonsialylated gonococci (50.7 versus 25.9% survival at 30 min). This decrease in opsonophagocytosis after sialylation may contribute to the pathogenicity of gonococcal infections.