Serial passage of mumps virus in the presence of inhibitory quantities of the capsular polysaccharide of Friediänder bacillus type B results in the appearance of a variant strain of the virus. Multiplication of the variant virus is not inhibited by the polysaccharide. A similar resistant variant is obtained with polysaccharide in a single cycle of multiplication when very large inocula of mumps virus are employed. The resistant variant is indistinguishable from the parent strain as to infectivity, reactivity with erythrocytes, and immunological properties, but appears to have a somewhat slower rate of multiplication. Serial passage of the resistant variant in the absence of polysaccharide results in the reappearance of a sensitive strain. It is suggested that mumps virus populations are inhomogeneous; that naturally occurring variants are present in such populations and possess distinctive properties; that the use of a chemical inhibitor of mumps virus multiplication makes possible the selection of a variant possessing a predictable property. The findings are discussed in relation to the mechanism of inhibition of mumps virus multiplication by polysaccharide.

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