SUMMARY: The growth of phage λ.C (i.e. phage λ grown in c) in K lysogenized by phage P1 is normally restricted so that the efficiency of plating of phage λ.C on K (P 1) compared to C is about 10. When K (P 1) bacteria are heated before infection with phage λ. C this restriction may be decreased as much as a million-fold. The time of exposure to elevated temperature (49° and above) required to achieve this increase in e.o.p. of phage λ.C decreased with increasing temperature up to temperatures which began to inhibit the capacity of bacteria to grow phage. Heated K (P 1) bacteria recovered their ability to restrict phage λ.C following the resumption of growth at 37°. Part of this recovery can be inhibited by chloramphenicol. A more dramatic recovery of restriction is observed when heated K (P 1) bacteria are resuspended in hypertonic media. Experiments are described which indicate that phage λ.C is restricted at an early step after adsorption and that, if phage escapes this restriction, it can grow in heated bacteria subsequently converted into restricting hosts by resuspension in hypertonic media.


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