The study aimed at morphological and immunohistochemical analysis of 53 mast cell tumors in dogs. Immunohistochemical studies included determination of cell markers for CD2, CD25, mast cell tryptase and mast cell chymase in canine MCTs and comparison of their expression levels with parameters of malignancy (grade, mitotic index, Ki-67 labelling index and CD117 staining pattern). Patnaik grading system identified 20 dogs with grade I MCTs, 17 with grade II and 16 with grade III. According to Kiupel grading, 34 dogs had low-grade MCT and 19 high-grade tumors. Mitotic index was between 0 and 2.4 (mean 0.4). The CD2 expression was high in 32 of 53 cases, CD25 in 20, tryptase in 44, and chymase in 15. The Ki-67 labelling index ranged between 0 and 55% (mean 11%). Majority of MCTs revealed the second staining pattern of CD117 (n=21). We found that expression of CD2 is linked with tumor malignancy, while CD25 determined tumor development. The tumor division into MCT_T and MCT_TC may be used as prognostic factor. MCT tryptase positive and chymase positive are less malignant than tryptase positive and chymase negative ones.