Antibody to canine parvovirus (CPV) was transferred from an immune bitch to her pups through the placenta and colostrum. Colostral transfer accounted for approximately 90% of the maternally-derived CPV antibody. After suckling, pups and hemagglutination-inhibition titers that averaged 50% of their dam's titer. Maternally derived CPV antibody declined with a half-life of 9.7 days. Pups with hemagglutination-inhibition titers greater than or equal to 1:80 were immune to oronasal challenge with virulent CPV, but any detectable hemagglutination-inhibition antibody (titer greater than or equal to 1:10) interfered with active immunization by modified-liver feline panleukopenia virus, inactivated feline panleukopenia virus, or inactivated CPV vaccines.