A study was carried out to evaluate, in cattle, the efficacy of a killed Myxovirus parainfluenza 3 virus vaccine which was incorporated into an emulsified mineral oil adjuvant. The vaccine was highly active in eliciting neutralizing, HI and CF antibody following a single dose in bovines and in preventing infection following artificial challenge by the respiratory route. By contrast, a prior history of one or two artificially or naturally acquired infections with the virus did not prevent reinfection on challenge. Appraisal of the value of the vaccine in preventing the clinical epidemiologic entity “shipping fever” must await clarification of the etiology and host factors responsible for this syndrome and proof of efficacy under natural conditions in the field. Evidence was presented for natural infection with Myxovirus parainfluenza 3 virus among initially seronegative calves in isolation. The source of the infection was unknown but might have been of human origin or have resulted from activation of a latent infection. The hemagglutination inhibition test was more sensitive than the neutralization or CF techniques for detecting antibody against Myxovirus parainfluenza 3 virus.