The diagnosis of extracutaneous sporotrichosis often presents a serious problem. This disease often mimics other mycotic and bacterial infections. A simply prepared complement-fixation antigen was developed and examined by screening a large number of known cases of sporotrichosis. This proved to be very useful and specific for the serologic diagnosis of extracutaneous sporotrichosis. No false positive reactions were observed when the antigen was allowed to react with sera from patients having other mycotic infections. The simplicity of the preparation and the specificity of the antigen allows it to be used in the hospital laboratory for rapid, accurate diagnosis of a rather atypical sporotrichosis.