The authors determined the optimal conditions for the lysozyme activity test to identify pathogenic staphylococci from various points of view. Besides, in an attempt to apply this test as a routine method, the usefulness of a modification of this test was checked as to a number of staphylococcal strains freshly isolated from human clinical materials. These strains were for interrelationships among coagulase, deoxyribonuclease, and other biological attributes. The results obtained are summarized as follows.1) The optimal conditions were determined for checking the lysozyme activity of staphylococci by means of the agar plate method. For this purpose was used a plate of ca. 15ml of “lysozyme agar”, which consisted of heart-infusion agar (Eiken), 40g; acetone-dried cells of the Miccrococcus lysodeikticus ATCC 4698 strain, 1g; and distilled water, 1, 000ml, and which had been adjusted to pH 7.1±0.1. It was streaked, ca. 1cm long, with one loopful of the test organisms and incubated overnight at 37°C. After an incubation at 37°C for 24 hours was made the judgment. When a transparent zone was observed obviously around the colony formed, the test was regarded as positive, or the test strain was regarded as a lysozyme-producer. It was confirmed that under such conditions the zone was the clearest and widest.2) By using this method, 509 strains isolated freshly were examined for lysozyme activity. As a result, a considerably good correlation was seen with coagulase production. Of 313 coagulase-positive strains, 349 were lysozyme-positive (98.9%). Of 153 coagulase-negative strains, 34 were lysozyme-positive (21.8%). This result was almost the same as that given by the phenolphthalein phosphatase test. It was a little inferior to that given by the DNase test, which showed such a close correlation as that obtained in the preceding investigation. The gelatin liquefaction, mannitol fermentation, and egg-yolk test were, however, considerably low in correlation with the coagulase test. This was especially the case with the coagulase-negative strains.3) The grades of enzyme activity of the test strains were measured by the turbidity method and expressed in terms of lytic index. Fifty strains each of Staphylococcus aureus and S. epidermidis were selected arbitrarily from the above strains. All the coagulase-positive strains were over 30 in lytic index. All the coagulase-negative strains were less than 20, except 3 strains which were a little higher.Thus, it may be concluded that the lysozyme activity test is useful, as a routine method, for identification of pathogenic staphylococci, especially when carried out in combination with the coagulase and DNase test.