Abstract: The method and apparatus for testing the susceptibility of bacteria to antibiotics described in the U.S. Pat. No. 3,832,532 and related patents are adapted for identifying various strains of bacteria by determining the light scatter indices (LSI) for a special group of antimicrobials, which have particularly significant characteristics with respect to identifying the strains of microorganisms, and analyzing them by a computer method, such as a quadratic discriminant function statistical technique, to identify the microorganisms to which the antimicrobials are applied. It has been found that 14 antimicrobial agents provide an extremely reliable identification and it is believed that the identifying group of agents could be reduced without sacrificing too much reliability. The agents preferably should be those which are not in common therapeutic use to avoid errors resulting from strains which have become immune to various therapeutically-utilized antibiotic agents.