Two evaluation procedures for efficacy testing of modified canine parainfluenza virus master seed within the restrictions of the seed-lot principle test are described. It is concluded the evaluation by post-mortem examination for specific hemorrhagic lung lesions more nearly reflects the objective of the seed-lot principle test method than does reduced days of shedding virulent virus after challenge. The problems associated with the seed-lot principle test method when working with diseases which are difficult to reproduce or modified viruses which show a dose response are discussed.