The presence of herpesvirus of turkeys (HVT) DNA in the feather tips of chickens vaccinated with HVT was assessed by dot blot hybridisation with a probe specific for HVT and lacking homology to MDV DNA. Only small amounts of HVT DNA were detected in the feather tips of chickens that were vaccinated or left in contact with HVT vaccinated chickens. However when chickens were challenged with virulent MDV, HVT DNA was detected in the feather tips of vaccinated chickens and the largest amount was detected 35 days after vaccination. HVT DNA was recovered in significantly higher quantities from some of the MDV-infected chickens than from those infected by contact. This suggests that MDV infection may provide helper functions for HVT. MDV DNA was identified in the feather tips of MDV-challenged chickens from 25 to 45 days after challenge. Thus, immunisation of chickens with HVT did not prevent the replication of MDV in the feather tips but only diminished it.