A serum-susceptible, guinea-pig chamber-passaged, laboratory strain (BS4 (agar)) of Neisseria gonorrhoeae was converted to serum resistance by incubation with cytidine 5-monophospho-N-acetyl neuraminic acid (CMP-NANA) and examined by electron microscopy after staining with ruthenium-red. In contrast to serum susceptible gonococci incubated without CMP-NANA, the majority (60-70%) of the serum resistant organisms showed a surface accumulation of polysaccharide. This surface polysaccharide was enhanced on all the resistant gonococci after incubation with fresh human serum. Control susceptible gonococci were devoid of the polysaccharide after incubation with heated human serum. Identical results were obtained with a fresh gonococcal isolate which had lost serum resistance on subculture but which, in common with 3 other isolates, was restored to serum resistance by incubation with CMP-NANA.