1. A factor necessary for the formation, of Staphylococcus muscae phage was found in acid digests of many highly purified proteins. 2. The factor is released from egg albumen and pepsin by peptic digestion. 3. No amino acids tried could replace the acid digests of proteins as a source of the factor. 4. The factor, when added to a multiplying bacteria-phage system, cannot be found in purified phage or in the lysate after complete lysis of the system has taken place.