Isolates of sheep pox and goat pox from Nigeria, Sudan, Kenya, Yemen Arab Republic, Turkey, Pakistan and India were inoculated into British breeds of sheep and goats. Although the isolates displayed a host preference the gross clinical pathology of the disease produced by the different isolates was indistinguishable. The Yemen, Nigeria and India isolates could not be distinguished using homologous and heterologous antisera in neutralisation tests. Animals that had recovered from infection with one isolate were resistant to challenge with any of the other isolates and a single vaccine for use against sheep pox and goat pox is described. The classification of the malignant pox diseases of sheep and goats is discussed.