The mechanisms by which ribonuclease and chymotrypsin inhibit the multiplication of a Rhizobium bacteriophage in liquid cultures were studied. Ribonuclease inhibited the multiplication of the host cell, had no effect on the free phage, but inhibited the interaction between phage and host. Chymotrypsin had no effect on either the free phage or the host cells, and it inactivated the phage only when the phage was combined with the bacteria. From the results observed when both ribonuclease and chymotrypsin were present, it is postulated that phage and bacteria do unite very briefly in the presence of ribonuclease, and during this combination the phage can be inactivated by the chymotrypsin.