The apparent lytic effect of a newly formed (‘nascent’) streptococcal bacteriophage on streptococcal strains not normally lysed by the phage was shown to be actually an inhibitory action produced by high concentrations of the phage. The inhibitory effect was exhibited by stock preparations of the phage, used in sufficient concentration, and there was no need to postulate a special potency for the phage in its ‘nascent’ form. The inhibitory effect of phages on heterologous organisms was found to be a widespread phenomenon among strains of Streptococcus cremoris and Str. lactis. It became apparent only when larger proportions of phage than the traces normally used for lytic tests were employed.