Shafia, Fred (University of Nebraska, Lincoln), and T. L. Thompson. Isolation and preliminary characterization of bacteriophage phimu-4. J. Bacteriol. 87:999-1002. 1964.-Bacteriophage phimu-4 was isolated from lysogenic Bacillus stearothermophilus NU strain 4, and was propagated in strain 10 of the same species. The phage was extremely host-specific. One of the 23 strains of thermophiles screened for susceptibility supported phage replication. Plaque-forming efficiency of phimu-4 depended on the agar medium employed and the temperature of incubation. Generally, media which resulted in restricted growth of the host cells on agar plates were most satisfactory for plaque formation. A latent period of 35 min was terminated in cell lysis and release of about 175 plaque-forming units (PFU) per infected cell. The phage particles, propagated in cells cultured in Trypticase (BBL)-yeast extract-dextrose-calcium chloride broth, were routinely removed from the lysate by 0.6 ammonium sulfate saturation in the cold. Phage purification was by ultracentrifugation followed by sucrose density-gradient. The phage particles appeared spherical, in electron micrographs. Particle size determination by electron micrographs indicated a diameter of less than 100 A.