Abstract
The antigenicity of two parainfluenza=3 virus strains, a "neuraminidasestrong" and a "neuraminidase-weak," was compared. For both strains the amount of hemagglutinin units was equal. The antibody responses to neuraminidase and hemagglutinin were measured on samples of serum and nasal secretion and were found to be similar, irrespective of the strain used for immunization. Anti-neuraminidase activity was demonstrated in the gel phase of nasal secretion of immunized cattle. Immunglobulin A was found attached to the peplomers of inhibited virus by immuno-electron microscopy.