Abstract
The plaque titre of rabbitpox and early passage vaccinia viruses was enhanced by diethylaminoethyl-dextran. Both viruses have low initial plaquing efficiency. Later passage vaccinia virus of high plaquing efficiency was little enhanced either before or after exposure to u.v., X or γ rays or to heat at 56°. The dextran made an insignificant improvement in adsorption of the virus particles to the cells. It formed flat, round bodies visible in the electron microscope but these were not found to form complexes with the virus particles nor to induce them to aggregate. This eliminated synergistic infection by aggregation of virus particles as a mechanism for plaque enhancement by dextran but suggested a protective action of the dextran particles upon freshly uncoated DNA within the cell vesicle.