AbstractThe inactivating efficacy for the typing phages 54, 83A, and 187 of the peptidoglycan ofStaphylococcus aureusis decreased by increasing the Ca++‐content of the medium; the irreversible inhibition of phage 187 becomes reversible. The percentage of its inhibition is proportional to the Ca++‐content of the medium. Minute amounts of phage 187 bound irreversibly under Ca++‐deprivation can be loosened by addition of Ca++‐ions. Addition of EDTA up to the equivalence concentration of Ca++present in the medium has no influence. Addition of EDTA up to the tenfold equivalence causes a significant increase of the inactivating efficacy of the peptid