A non-specific deoxyribonuclease with a possible role in the restriction of some actinophages was detected in Streptomyces glaucescens ETHZ 22794. Production of this enzyme activity was influenced by the medium composition, indicating nutritional control of enzyme synthesis. Restriction was confirmed when phage adsorption and efficiency of plating in nuclease-productive and non-productive media were investigated, and also by analysis of a mutant which lacked exonucleolytic activity. In vivo escape from restriction in nuclease-productive media is mainly related to the ability of phages to adsorb in a growth phase earlier than that in which enzyme synthesis occurs.