An extensive characterization of plasmid-dependent phage PR5 isolated from sewage has been carried out. The phage has a head diameter of 65--68 nm, is isometric with a double-layered capsid, and a minority possess tails. It adsorbs to many but not all types of bacteria possessing P, N, or W plasmids. The phage contains 20% lipid, 15.1% DNA, and 64.9% protein by weight and has a buoyant density of 1.265 g/ml in CsCl. The DNA is double-stranded with a G + C content of 49% and a molecular weight of 7.4 +/- 0.6 x 10 (6) as shown by electron microscopy. Phospholipid content is 66% of lipid and consists of cardiolipin (13%), phosphatidylethanolamine (43%), and phosphatidylglycerol (44%) and differ quantitatively from that of host bacteria. Anti-PR5 serum inactivates other similar phages, PR3 and PR4. Phage adsorption is impaired in deep rough mutants of Salmonella minnesota.