Summary: Various strains of which type exclusively with phages of lytic group II were found to modify phage K so that its ability to form plaques on host was lessened. The restricted phage formed plaques with high efficiency on all strains of lytic group II. In general, it plated at lower titres on strains of lytic groups I, III, IV, and on some strains of miscellaneous typing characteristics; however, there were some variations among separate cultures of the same strains. For example, the restricted phage plated at high titre on strains 52/79, 73, and 44, but formed significantly fewer numbers of plaques on strain 52/79 and on a second culture of 44 Strain was found to dissociate into apt () and non-apt () forms. The probability of plaque development by restricted phage on strain was dependent upon the nutritional state of the cocci and also upon the proportion of apt and non-apt cells. The restriction of phage K was eliminated during its propagation on all strains other than lytic group II. The unrestricted progeny, particles tended to assay at equal titre on all the indicator strains. In all cases the genotype of the phage-susceptibility to host-control-remained unchanged. The observations add to existing data which indicate that strains of phage group II form a genetically distinct group. The suggestion is made that this phenomenon might help in taxonomic classification of strains of


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