A polylysogenic culture of Actinomyces griseus 15 producing the antibiotic cormogrisin is sensitive to actinophages, denoted as types 15 I, 15 II and 15 III, which are the virulent mutants of its temperate phages. In certain cases, the variants of Act. griseus 15, obtained by selection and resistant to the phages types 15 I and 15 II, can undergo lysis-from-without by these phages. A study of the mechanisms involved in this phenomenon has shown that the lytic activity is entirely eliminated if the phages have been a titre of 10(9) and higher. The lytic activity is entirely eliminated if the phages have been treated with a specific serum or inactivated by UV. The variants resistant to phage 15 I can adsorb this phage and undergo lysis-from-without at a high multiplicity of infection if only the age of the variant does not exceed 13 hours. The variants resistant to phages 15 I and 15 II can undergo lysis-from-within under the action of phage 15 III. The variants resistant to this phage have acquired this resistance as a result of lysogenization. They have entirely retained the ability to adsorb phage 15 I, but their ability to absorb phage 15 III is much lower. Therefore, the rate of mycelium growth is only slightly inhibited, but the lysis of the culture is not observed, under the action of phage 15 III having titres of 10(9) and 10(10).