Soluble group antigen isolated from bluetongue virus infected BHK cells has been shown to be a single protein with a molecular weight of 38,000 and to comigrate on polyacrylamide gels with protein 7 isolated from virus particles. Purification by exclusion chromatography and isoelectric focussing yields a protein that appears to be antigenicaly identical to the native group antigen found in virus harvests; it is non infectious for sheep and is thought to be largely free from host cell and other virus specified proteins. It is suggested that this antigen could be used in diagnostic procedures with safety.