The ubiquitin-immuno-reactive protein with a molecular weight of 27,800 daltons, which is mainly present in the cap of young basidiocarp, was purified from the basidiomycete Coprinus cinereus. The molecular weight of the native protein was approximately 55,000 as determined by gel filtration chromatography. The isoelectric point of the protein was 4.4. The amino-terminal sequence of the protein was also determined.