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Mast cell tumours in dogs are often underestimated by clinicians concerning their malignancy. In more than one half of the examined cases the diagnosis of systemic involvement was made by the pathologist. Hence it follows the need of a routinely practicable examination for a better classification of malignancy, i.e. the risk of metastasis of this kind of tumour.Twenty-four systemic and thirty-one solitary archivated cases were examined.On each tumour specimen the proliferation index was evaluated by counting 1000 cells, in case so many tumour cells were available. This was done the first time by an immunhistochemicalenzymhistochernicaI double-staining procedure through the monoclonal antibody MIB 1 and he detection of the mast cell specific enzyme chymase.Furthermore, the histological grading system by PATNAIK et al. (1984) for mast cell tumours in stages from 1 to Ill (good to poorly differentiated), was modified and extended respectively.By the examination it was evaluated that mast cell tumours are mostly a disease of older dogs. Bitches are more often concerned at systemic illness whereas male dogs are more afflicted by solitary tumours. There is a disposition for poodles and poodle-hybrids for systemic and for boxers in solitary tumours. The occurence of paraneoplastic syndromes is an often seen feature. It is an evidence of the involvement of inner organs. It was seen in nearly half of the examined cases.The proliferation index increases with the grade of malignancy. Solitary grade-I-tumours show an average proliferation index of 2,0% whereas dermal tumours of the systemic diseases have an average proliferation index of 13, 1 %Dermal tumours of systemic mast cell illness conform to 78, 1 % to grade II and only 18,8% to grade Ill.Preferred organs of metastasis are the lymphonodes, liver and spleen.Other, former discussed features for mast cell tumour classification do not correlate with the proliferation index.The combination of the proliferation index of 5% or more with a histological grade II or higher now allows to assume with high probability that metastasis of a tumour will occur or has already taken place. A seek for metastases or a medical treatment could be earlier prepared as it is possible today.