AbstractThe growth and Cytopathogenicity of five strains of mumps virus were examined in six types of cell cultures and in neonatal hamsters. These strains included the MJ and RW strains, both recent cerebrospinal fluid isolates; the neuroadapted Kilham strain; the Enders strain adapted to chick embryo; and the Jeryl Lynn vaccine strain adapted to chick cell culture. The MJ, RW, and Kilham strains all produced infectious virus without restriction in vitro, but the RW strain did not cause obvious cytopathic effect; the MJ and Kilham strains were cytopathic. The Enders and Jeryl Lynn strains adapted to chick embryo cells were cytopathic and productive in chick cell culture but were restricted in ability to grow productively in vitro on mammalian cell types, even failing to produce noninfectious particles in some cases. In vitro Cytopathogenicity was a host‐independent property of a specific virus strain, but the type of cytopathic effect manifest in culture (eg, fusion, cytoplasmic vacuoles) depended on both the strain and the host cell. The ability of a virus strain to invade the brain parenchyma and infect neurons in vivo appeared to correlate with the strain's Cytopathogenicity and not with passage history or adaptive statu