Although it is difficult to ascertain the true protein content of mushrooms because of their variable nonprotein nitrogen content and protein heterogeneity, even among strains within a species, some knowledge about a mushroom's protein is essential for nutritional, processing and economic considerations. Protein similarities and differences in cultivated mushrooms are shown with the aid of three analytical methods. Protein data based on amino acid analyses are preferred but too expensive to obtain. Amido black dye binding is the next preferable method. Its correlation with amino acid protein was higher (D.74) than that between Kjeldahl protein and amino acid protein (D.44).