Two groups of six-week-old beagle puppies were vaccinated with either high titre canine distemper virus or human measles virus, a third group remaining unvaccinated. All the puppies were subsequently challenged by the nasopharyngeal route at 10 weeks old with the virulent Snyder-Hill strain of canine distemper. Severe clinical signs were observed in 90 per cent of the unvaccinated dogs but both groups of vaccinated dogs survived the challenge. High temperatures were recorded in 20 per cent of the measles vaccinates and abdominal petechial rashes were observed in 60 per cent of them. The only clinical signs observed in the puppies vaccinated with distemper virus were transient rashes in 20 per cent of the group. The high titre canine distemper vaccine stimulated a humoral response quickly in 78 per cent of the puppies in the presence of maternally derived antibody and protected them against challenge with the virulent Snyder-Hill strain of distemper virus. The remaining dogs responded sluggishly but were still protected against challenge. The results of field surveys showed that 95 per cent of young puppies with different levels of maternally derived antibodies responded to the distemper component in a vaccine also containing canine parvovirus. No incompatibility was observed between the two components.