Comparison was made of the properties of the bovine SF-4 and the human HA-1 strains of Myxovirus parainfluenza 3 virus. Both grew abundantly in tissue cultures of bovine and monkey renal epithelium, causing characteristic cytopathic change. Antigenic difference between the strains was demonstrated in serum neutralization, CF and HI tests. The SF-4 strain generally appeared to have an antigenic composition more broad than that of the HA-1 virus. The two strains also differed in that the SF-4 antigen agglutinated chicken erythrocytes at room temperature but the HA-1 virus failed to do so. Tests for presence of nonspecific inhibitors of hemagglutination in human, bovine and rabbit antisera against Myxovirus parainfluenza 1 and 3 viruses were negative. It was concluded that nonspecific inhibitors for the parainfluenza viruses tested were either absent entirely or of such low titer as to be without consequence in serodiagnostic and sero-epidemiologic studies with these agents. Complete correspondence in the diagnostic results of paired sera from cases of SF-4 virus infection in calves was obtained using the serum neutralization, CF and HI techniques.

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