A slide co-agglutination test (Phadebact® Gonococcus Test) for the serological identification of Neisseria gonorrhoeae was assessed on gonococcal-like, oxidase positive colonies from 120 cultures, originating from about 6,500 consecutive tonsillo-pharyngeal specimens received at the Neisseria Department, Statens Seruminstitut. The test was performed after subculture on a serum-free medium, since this procedure was found to reduce the number of strains showing inconclusive reactions (pseudo co-agglutination). If this pseudo co-agglutination does occur, however, the test can be repeated with the addition of trypsin to the test system. This causes the previously inconclusive reactions to be reverted to clearly positive reactions in the case of gonococci, and to clearly negative reactions in more than half of the previously inconclusive reactions with other bacterial strains. The results obtained by the Phadebact Gonococcus Test were compared with those obtained by bacteriological identification procedures. Fifty-six of the 120 cultures examined contained gonococci, and all strains were identified by the slide co-agglutination test (five strains with the addition of trypsin). The remaining 64 cultures were negative or exhibited consistently pseudo co-agglutination (eight strains). The specificity and sensitivity of the reagent was further confirmed by the examination of 53 strains of Neisseria gonorrhoeae and 50 strains representing Neisseria species commonly occurring in tonsillo-pharyngeal specimens. The Phadebact Gonococcus Test was considered to be a reliable alternative to routine bacteriological identification of Neisseria gonorrhoeae.