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It was observed by Hershey, Kamen, Kennedy, and Gest (1951) that bacteriophages are unstable if they contain radiophosphorus p82 of high specific activity. From day to day, progressively decreasing fractions of such populations of radioactive phage are still able to form plaques when plated on a sensitive bacterial strain, and the rate of loss of infective titer depends on the specific activity of the P= assimilated. It is the purpose of this communication to present experiments in which these observations of Hershey et al.have been extended to the study of the lethal effects of ps~ decay in various strains of bacteriophage at various temperatures and to the examination of some of the biological properties of the inactivated bacteriophage particles. Some of these experiments have already been reported in preliminary form . Materials and MethodsBacteriophages T1, T2, T3, TS, TT, and their host, E. ¢oli B/r, and phage )~ and its host, E. co~i strain K12S, were used in this study. Strain B/r, a radiation-resistant mutant derived from strain B, was kindly supplied to us by Dr. Aaron Novick.Glycerol-casamino acid medium refers to a medium devised by Fraser and Jerrel (1953). H medium is a glycerol-lactate medium of the following composition per liter of distilled water: 1.5 gin. KC1, 5 gln. NaC1, 1 gin. NH4C1, 0.25 gin. MgSO4.TH~O, 10 "4 N CaC12, 0.07 M sodium lactate, 2 gin. glycerol, 0.5 gin. bacto-peptone Difco and 0.5 grn. bacto-easamino acids Dffco. H medium contains 6 reg./liter total phosphorus, of which 5 reg./liter are supplied by the easamino acids and I reg./liter by the peptone. Control experiments show that this phosphorus is assimilated by cultures of E. coli neither more nor less readily than inorganic phosphate. The techniques described by Adams (1950) were employed for the general procedures of bacteriophagy.

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