Summary A quantitative study has been made of the serological properties of anti-X serum, prepared by injecting rabbits with beef serum coupled with diazotized@-aminobenzoic acid. It was found that this antiserum gives specific precipitation with a simple substance containing two p-azobenzoic acid groups (substance XXXV), made by coupling diazotized p- (p-aminopheny1azo) benzoic acid to chromotropic acid, but not with any one of six other ...