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Recent reports(3,S) indicate that the incidence of penicillin-resistant strains of Micrococcus pyogenes causing infections of man is increasing. Many of the factors responsible for the emergence of such a resistant population of micrococci are as yet unknown. It is a well established fact( 1,2,4), however, that the ability to produce the enzyme, penicillinase, is the primary factor in the natural resistance of these cultures of M . pyogeizes to penicillin. Of considerable interest in this respect is the report of Barber (6) indicating that penicillin-sensitive mutants may be isolated from cultures of peniciIlinase-producing micrococci. The sensitive mutants described by this worker did not produce the enzyme. The study reported in this paper was undertaken to investigate this phenomenon further in an attempt to determine its possible relationship to the problem of the increasing incidence of penicillin-resistant micrococci.Materials and methods. The strains of micrococci studied consisted of a group of penicillinase-producing cultures isolated from infections over a period of the past 5 years.Since there is no correlation between penicillinase-production and type of pigment ( 7 ) , we have not classified these strains as Micrococcus pyogenes var. aweus or Micrococcus pyogenes var. albus. All cultures were maintained on brain heart infusion (B.H.I.) agar slants covered with steriIe mineral oil. During the period of investigation, stock cultures of each strain were transferred monthly on B.H.I. agar slants and stored in the refrigerator. Unless otherwise noted, nutrient agar and broth were used exclusively throughout the study. During the early phases of this study, the method for detecting susceptible mutants was similar to that of Barber (6) with minor modifications. Cultures of penicillinase-producing micrococci

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