Strains of Neisseria gonorrhoeae grown on Thayer-Martin medium and media with and without serum were examined by the Phadebact Gonococcus Test. By using the direct method, in which colonies from growth media were mixed directly with the reagents on a microscope slide, Thayer-Martin medium, contrary to the manufacturer's claim, was not found to be the medium giving best results, and presence or absence of serum in the media did not affect the results. A large number of inconclusive results were obtained. A modification of the alternative procedure, which uses a heated suspension of the gonococcal cells as the antigen, is described. Of the 432 strains of N. gonorrhoeae examined by this method, 427 were accurately identified. Five strains, however, gave false-negative results. None of the 80 strains of N. meningitidis gave positive results. Nine of nineteen strains of N. lactamica, however, gave clear-cut positive results. Use of the o-nitrophenyl-beta-D-galactopyranoside test in conjunction with the Phadebact Gonococcus Test, particularly on isolates from the pharynx, is recommended.