Serotyping of 168 clinical isolates by the slide agglutination test using TIBS (Toshiba Institute of Biological Science) serotyping sera, which were prepared from Homma's serotype strains at the request of Pseudomonas aeruginosa Serotype Committee in Japan, was performed and the results were compared with serotyping using IMSUT (Institute of Medical Science, University of Tokyo) serotyping sera by the tube agglutination test. When heat-killed antigen from clinical isolates were used, both sets of serotyping sera showed similar results and the slide agglutination test using TIBS serotyping sera could be substituted for the tube agglutination test using IMSUT sera. It was also discovered that serotyping by the slide agglutination test with TIBS serotyping sera could be carried out using live bacteria. However, in serotyping with live bacteria, there is some difficulty in observing the agglutination time. To solve this problem, we propose that positive agglutination should be determined within 60 sec. The authors also performed serotyping of the clinical isolates using Difco serotyping sera prepared by the Difco Company based on Liu's serotyping system, and the results were compared with those obtained with serotyping sera prepared with Homma's serotype strains. The results indicated that in serotyping of clinical isolates using Difco serotyping sera more cross reaction occurred than in serotyping with sera obtained from Homma's serotype strains.