The morphological heterogeneity of the chick embryo-adapted Enders strain of mumps virus was examined in relation to biological and biochemical properties of the virus. The range of virion sizes increased as multiplicity of infection (m.o.i.) increased, with virions of 800 to 1000 nm in diam. occurring in preparations grown at the higher multiplicities. There was no evidence for a distinct class of non-infectious haemagglutinating particles. Dense fractions from isopycnic gradients of virus were enriched in larger virions, but virus RNA was predominantly a single 50S species. Despite evidence that many virions were multiploid, u.v. inactivation kinetics were single-hit, suggesting only a single biologically active genome per virion.
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