Benyesh-Melnick, Matilda (Baylor University College of Medicine, Houston, Tex.), Fern Probstmeyer, Robert McCombs, Jean P. Brunschwig, and Vladimir Vonka. Correlation between infectivity and physical virus particles in human cyto-megalovirus. J. Bacteriol. 92:1555-1561. 1966.-Infectivity titers [measured as plaque-forming units (PFU)] and particle counts by the sedimentation pseudo-replication technique were determined for crude, unpurified, intracellular preparations of two different strains of human cytomegalovirus. Unlike the high particle-infectivity ratio of 10(6) to 10(8) previously reported for these viruses, the number of total particles per PFU ranged from 160 to 490 with strain AD-169 and from 176 to 1,050 for strain C-87. Interpretation of particle-PFU ratios of intracellular cytomegalovirus in terms of particle morphology is not conclusive at this time. The number of enveloped forms found varied between 0 and 34% of the total particles counted. However, the true proportion is probably greater, because envelopes were found to be destroyed by the enzyme treatment used in preparing the specimens for examination in the electron microscope. The number of full particles found ranged between 4 and 31% of the total particles counted. The particle per PFU ratio of extracellular virus was found to be three- to fivefold lower than that of intracellular virus.