Abstract
T3 hs phage is a strain of T3 h phage which produces an irregular plaque on E. coli B wild type strain, but the round one on E. coli Bs/3 strain. The experiments reported here is performed to secure the mechanism of irregular plaque formation on E. coli B by this phage. It was concluded that the T3 hs phage is a minute mutant (T3 hm) which produces a minute plaque only on E. coli B and a normal round one on E. coli Bs/3 strain in nature, but the character is so unstable on E. coli B that it easily mutates back to T3hm+. If the mutation occurrs at an early stage during the development of T3 hm phage, the irregular plaque which has the protruded area where the phage mutated back is formed.
The existence of a minute mutant recognizable only on E. coli B in which mutation from m to m+ occurs so frequently and so easily that it forms an irregular plaque, is shown in this study but it has not yet been clarified whether the mutation occurs by the recombination between the bacteria and phage or only by a spontaneous mutation i.e., a mutable gene.
