Velocity sedimentation at high ionic strength, of DNA released from M13-infected bacteria lysed under gentle conditions, provides a simple method for separating phage-specific DNA components from bacterial DNA. 1. 1. Components sedimenting at rates of 0.71 and 0.57 times that of viral single strands are identified as component I replicative form (RFI-DNA) and component II replicative form (RFII-DNA), respectively. 2. 2. The major phage DNA component early (<30 min) in the infectious process is RFI-DNA; at later times (> 45 min) the major phage DNA component is intracellular single strands. 3. 3. The source of intracellular single strands is a pool of free DNA molecules rather than progeny phage particles. © 1969.