The rate at which cultures of B. megatherium 899a produce certain mutants of its phage T (wild type) has been investigated in two media; one (peptone) in which reinfection of sensitive cells, if present, is virtually certain, and one (asparagine) in which reinfection is extremely unlikely. Little difference either in the number or types of mutants produced has been detected.

It is concluded that phage mutants are produced directly by B. megatherium 899a and that no intermediate passage through a sensitive cell is necessary for the mutation.

396 individual mutant plaques have been isolated and classified into at least 18 types of plaque-forming mutants of B. megatherium 899a.

The over-all mutant ratio found was in the order of 1:2000 while individual mutants appeared with rates between 1:7000 and less than 1:100,000.

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