The number of mast cells in the stroma of human squamous cell cancer -- keratoblastif cum, paraepidermale and male diffusa was 2-30 times lower than in the stroma o-normal squamous epithelium or in the connective tissue distant from neoplastic epithelium. These differences are statistically highly significant. The reduction of mast cel, number apparently might be the result of exhaustion of heparin-containing granules which interfere with tumor growth.