Abstract
A flock of 33 lambs born to ewes with neutralizing antibody to parainfluenza type 3 (PI3) virus was run with a flock of approximately 150 other lambs and their mothers. When bled shortly after birth, lOO% of the 33 lambs had maternal anti-body to PI3 virus. Four months later, the proportion h!lod fallen to 22%, about which time a natural infection with PI3 virus occurred in the flock as judged by subsequent appearance in the sera of PI3 serum neutralizing antibody.

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