SummaryTwo strains of human rhinovi-ruses, B 632 and Baylor 1, have been purified and concentrated by fluorocarbon extraction, ultracentrifugation, and cesium chloride density gradients without significant losses in titer. Biophysical studies indicate that they have a buoyant density in cesium chloride of approximately 1.4 g/cm3 while acid-insensitive picornaviruses (enteroviruses) have a buoyant density of approximately 1.34 g/ cm3. Measurements of electron micrographs show that the virions of Baylor 1 and B 632 are 20-23 mμ in diameter.
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