1. The seven bacterial viruses of the T group, active against E. coli, are much more rapidly inactivated by heat when suspended in 0.1 N solutions of sodium salts than when suspended in broth. 2. The kinetics of this inactivation whether in salt solutions or in broth are those of a first order reaction. 3. The rate of inactivation of phage T5 in 0.1 N NaCl at 37 degrees C. can be greatly decreased by the addition of 10(-8)M concentrations of such divalent cations as Ca, Mg, Ba, Sr, Mn, Co, Ni, Zn, Cd, and Cu. 4. An increase in the cation concentration in the suspending medium results in an increase in the stability of phage T5 to the inactivating effects of temperature. 5. The hypothesis is proposed that the increase in stability of phage T5 in the presence of various cations is the result of complex formation between the phage and the metal ion.