The aim of the study was an evaluation of the activity of mast cells and mark activity tryptase and chymase and of protein levels in experimental fibrosarcoma, induced in rat skin. The experiments were carried out on 50 male Wistar rats. The cancer was induced in rats by one subcutaneous injection of 0.2 mg 3-methylcholanthrene in 0.25 ml of olive oil. Tissue material was fixed in Bouin's fluid. Immunohistochemical tryptase detecting reactions were performed--using specific antibodies and the ABC complex. The activities of tryptase and chymase and protein levels were determined in supernatant of 10% homogenate. We found a very significant growth of mast cell quantity in the connective tissue of tumours. We observed slight differences in the activity of examined enzymes in tumours of different mass.