ECHO 6′ virus produces plaques of 1-2 mm in diameter on MS cell monolayer. The large plaque mutants of 6-10 mm were obtained from the virus passaged in the presence of anti-ECHO 6 serum. This paper compares the various characteristics of the mutants and the small plaque formers including the original ECHO 6′ virus strain.The large plaque mutants were more sensitive to anti-ECHO 6 and 6′ sera than small plaque formers. The mutants began to grow earlier and produced more virus particles than small plaque formers.The effects of certain drugs on plaque formation were examined. MgCl
2 and cysteine had no effects except on certain strains. Dextran sulfate promoted the plaque formation in all the strains. A low concentration of sodium bicarbonate promoted the plaque f of mation only in the small plaque formers.The original strain and one small plaque former had the ability of hemagglutination, but one small plaque former and all mutants did not.Comparing the above characteeristics, it is ascertained that the plaque mutation linked to antigenic character but not necessarily to the other characterisitics.
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