Some biological properties of two strains of turkey rhinotracheitis virus, isolated 10 years apart in South Africa, one from turkeys and one from chickens, have been compared. Whilst in vitro cross-neutralization tests showed them to be closely related antigenically they showed different in vivo properties. Both strains elicited an antibody response and caused clinical signs of infection in both chickens and turkeys although the signs tended to be more marked in the species from which the TRTV had been isolated initially. Replication of each virus strain occurred principally in the upper respiratory tract with little virus being recovered from other tissues. The chicken isolate replicated to very high titre (approximately log(10) 6.0 median ciliostatic doses of virus per g) in nasal tissue of both chickens and turkeys, whilst the turkey isolate was only recovered in such large amounts from nasal tissue of the turkeys. There appeared to be little difference in the clinical signs which the chicken isolate caused when seven different inbred chicken lines were inoculated intranasally with it.