Vaccinia virus, inoculated at a multiplicity of 10 particles per L cell in closed tube cultures with media containing the serum of certain individual horses has produced progeny notably poor in plaque-forming efficiency (quality). The total virus particle yield from these cultures has been high and independent of the particular serum used. These differences were greatly reduced when the sera were heated at 56°C for 30 min. Possible reasons for these effects include indirect influence of a heat-labile serum component upon progeny virus quality through the agency of the cell.

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