Recent studies have demonstrated that virulent (Types 1 and 2) colonial variants of Neisseria gonorrhoeae are highly resistant to phagcytosis by normal human polymorphonuclear leukocytes. To pursue this observation, sera from prostitutes were compared with sera from Catholic nuns in terms of their ability to enhance phagocytosis of Type 1 gonococci. The phagocytic system consisted of a monolayer of normal human polymorphonuclear leukocytes and an inoculum of 107 to 108 colony-forming units of N. gonorrhoeae. In a representative test, the mean percentages of phagocytosis were: 13 nuns, 10 per cent; 32 prostitutes, 30 per cent (p less than 0.01). In a separate set of experiments, 5 (45 per cent) of 11 patients with acute gonococcal salpingitis had significant increases in serum opsonic activity between the acute and one month convalescent bleeding. A pool of prostitute sera retained detectable opsonic activity at dilutions as high as 1:320. The opsonic activity was heat-stable, demonstrable without complement, and resided in both the IgG and IgA fractions of serum. These studies demonstrate the presence of acquried opsonic antibody to N. gonorrhoeae in humans: the significance of such antibodies in proction against gonococcal disease requires further exploration.