Two groups of puppies, one passively immunised by the administration of hyperimmune serum and the other with natural maternally derived antibody, were inoculated orally with virulent canine parvovirus of faecal origin. Serum antibody titres declined more rapidly in both groups after challenge than before. The dogs became clinically affected but the onset of clinical signs, seroconversion and faecal excretion of virus was delayed when compared to controls. It is postulated that this rapid decline of antibody was due to its sequestration by virus after the initial phase of viral replication in the lymphoid tissues. These findings have important implications. The incubation period of the disease is prolonged, making it more difficult to estimate accurately the time of infection in clinically affected animals. Furthermore, the more rapid decline of maternally derived antibody, which could occur in endemically infected premises, may complicate immunisation programmes based on the isolation and segregation of puppies in anticipation of a predicted decline in maternally derived antibody before vaccination.