Plaquing of a newly isolated phage of Bacillus subtilis, phage 41c, is only 2% efficient in agar containing 200 mug of deoxyribonuclease per ml. Timed deoxyribonuclease addition experiments showed that phage development is blocked in 90% of the cells if deoxyribonuclease is present during adsorption (zero-time samples), whereas 10 min after adsorption the enzyme has little effect (10-min samples). The fate of (32)P-deoxyribonucleic acid label of phage 41c in zero-time samples was compared to that in 10-min samples. In both, about 80% of the label remained with the phage-bacterium complex on initial centrifugation. However, four successive washings removed 90% of the (32)P from the zero-time samples but only 25% from the 10-min samples. In both samples, most of the washed-out label was of low molecular weight. When the time course of interruption of infection by blending was compared with interruption by deoxyribonuclease treatment, the two processes exhibited similar kinetics. It is postulated that both processes block injection at the same site, namely, the point of contact between phage tail and cell wall surface. Partitioning of (32)P label during protoplasting of zero-time and 10-min samples was similar to that observed during washing. For the protoplasting experiments, a quantitative method for plaquing protoplasts was developed. A single bacillus made of several cells can give rise to several protoplast plaque-forming units. Strain 41c was the only phage of seven tested to be inhibited by deoxyribonuclease. No other deoxyribonuclease-sensitive phages have been described.