Analysis of phagolysates in production of fodder grisin provided determination of 3 serologically and morphologically different actinophages, 2 of which, i. e. Pg 81 and Pg 100 affected the sensitive culture as virulent phages and 1, i. e. Pg 2 affected it as a moderate phage. Investigation of the fermentation broth of strain 15 of Act. griseus did not confirm the supposition that this strain was polylysogenic and actinophages Pg 81 and Pg 100 were virulent mutants of moderate phages present in the cells. A phenomenon of limitation and modification controlled by the host cell was noted in the sensitive culture infected with actinophage Pg 81. The actinomycete mutants which did not limit but modified the phage were isolated. Under the effect of chemical mutagens the actinomycete mutants stable to virulent phages were selected. Treatment of the phage-stable mutants with ethylene imine and UV-light provided an increase in their antibiotic production levels by 75 to 80 per cent.