The Infectivity of Purified and Partially Purified Preparations of Vaccinia and Cowpox Viruses

SUMMARY: Twenty-three preparations of vaccinia virus and 5 of cowpox virus were examined by electron microscopy of spray droplets and by infectivity titrations at various stages of purification. Extraction of virus-containing tissues with a fluorocarbon compound was a useful early step in purification, but preparations of the highest purity were obtained by flocculation of the virus with m-NaCl. The average ratio of total particle count to infectivity for the vaccinia virus preparations was 12. The lowest observed ratio based on reliable particle counts was 3·4.