A five-year-old labrador bitch which had whelped 10 pups three days previously was given booster vaccination against distemper, adenovirus, parvovirus, parainfluenzavirus and leptospirosis. Eighteen days later, signs of central nervous system disease developed in some of the pups, five of which were ultimately euthanased. The cause of the nervous disease was found to be canine distemper, and serological studies showed that the infection was limited to some members of the litter, suggesting that the vaccinal rather than a field virus was more likely to have been responsible.