ABSTRACTA biologically active peptido‐rhamnogalactan was isolated from disruptedSporothrix schenckiiyeast cells by sodium deoxycholate extraction and was further purified by DEAE‐Sephadex A‐50 chromatography. This fraction, 60.6% sugar and 3.0% nitrogen, was active in delayed hypersensitivity skin tests in sporotrichosis patients and also cross‐reacted serologically with a peptido‐rhamnomannan isolated fromSporothrix schenckiiculture filtrates. These results suggest strong that L‐rhamnose, α‐1, 2 linked, is more important in determining the serological specificity ofSporothrix schenckiithan the α‐1, 3 linked L‐rhamnose or the mannan backbone of the peptido‐rhamnomannans so