The buoyant density characteristics of infectious particles of herpes simplex virus types 1 and 2 were studied by centrifugation in sucrose and cesium chloride density gradients with a high resolution and satisfactory infectivity recovery. It was shown that two populations of infectious virions differing in buoyant density coexisted, the difference being slight but definite. The ratio of heavy (H) to light (L) viral particles varied depending upon the solute used, the strain of virus, and the

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... ll origin. Circumstances favoring degradation of viral infectivity tended to increase the H portion. Incubation at 37°C largely converted L to H, and heating at 450C converted all virions to H without infectivity. The L to H conversion was irreversible, and no populations intermediate between L and H were clearly observed. Inactivation by UV light irradiation did not affect the density pattern. That H was not an artefact due to penetration of solutes, osmotic pressure, viral aggregation, or loss of the envelope was shown experimentally. A difference in the outer shape of particles between negatively stained L and H populations was demonstrated by electron microscopy. Both cell-released and cell-bound herpes simplex virus particles gave essentially the same result with respect to the above characteristics. The effect of limiting dilutions of antiserum was similar to that of mild thermal treatment, in that denser virions increased parallel to a decrease in less dense virions. Sensitization with early immunoglobulin G, composed mainly of complement-requiring neutralizing antibody, caused the density transition, and subsequent addition of complement resulted in a further increase in the buoyant density of the sensitized virions. The DNA in virus particles neutralized with immunoglobulin G plus complement remained resistant to DNase treatment. Possible implications of the phenomena are discussed.