Two interesting new phenomena have been observed in suspensions of T4Bo bacteriophage which were frozen to temperatures below the eutectic temperature of the salt (sodium chloride) in the suspending medium. Approximately 10% of the phage appeared to survive such a phase change as determined by plaque titre. However, exposure of these survivors to ultrasonic vibration or repeated freezing showed them to be hypersensitive and thus latently injured. The hypersensitivity was lost on incubating the phage at 37° C. for 3 hr. Furthermore, following a eutectic phase change, the surviving phage could be inactivated by rapid cooling to — 90° C. followed by slow rewarming. Such inactivation cannot be accounted for by accepted theories of freezing injury.