Antibody titers measured by hemagglutination inhibition testing were determined in previously vaccinated dogs at the time of booster vaccination and 2 weeks later. All vaccines consisted of modified live panleukopenia virus. The booster injection was administered approximately 6 months after the initial parvovirus vaccination series was given. Fecal and serum specimens were collected immediately before and 2 weeks after administration of the booster vaccine for hemagglutination and hemagglutination inhibition testing, respectively. All dogs were privately owned and were from the Columbus, Ohio, area but were from environments with various exposure potentials to canine parvovirus. Results of hemagglutination (HA) testing on feces were negative in all dogs before and after booster vaccination. Therefore, these vaccinations did not interfere with interpretation of HA testing of feces. Results of serum hemagglutination inhibition (HI) testing indicated that 50% of the dogs had serum titers less than 1:80 prior to vaccination and that, of these dogs, 65.2% still had serum titers less than 1:80 2 weeks after the booster vaccination. Only 10.9% of all dogs had a marked increase in serum HI titer after the booster vaccination, indicating that overall serologic response to vaccination was poor. High HI titers (greater than or equal to 1:640) were associated with exposure to other dogs and cats in the neighborhood or to dogs suspected of having had parvovirus infection.