A soluble protein of unknown function was shown, by two-dimensional polyacrylamide gel electrophoresis, to be present in Neisseria gonorrhoeae at significantly higher concentrations than in the other related bacteria tested. The data indicate the possibility of this protein being specific to Neisseria gonorrhoeae. This protein was designated as NG 8.4 and purified. A radioimmunoassay was developed for the measurement of this protein and subsequently used to determine the degree of cross reaction exhibited by a number of species of bacteria. Of the bacteria tested only those of the genus Neisseria gave a significant reaction in the assay.