Tryptase has been considered as a cellular marker of mast cells in the tissues of human and some mammals. In order to detect whether or not fish mast cells contain tryptase in their cytoplasm, a murine monoclonal antibody (AA1), raised against human mast cell tryptase , was used to stain the paraffin sections of the digestive tract tissue collected from Tilapia nilotica，Anguilla japonica and Anguilla anguilla and gastric cancer tissue(as a positive control) from an adult man by Elivision^TMplus immunohistochemical techniques, Alcian blue Safranin O staining and the modified toluidine blue staining were used as histochemical staining at the same time. The results showed: Nile tilapia mast cells were displayed better by Alcian blue Safranin O staining and the modified toluidine blue staining, while the effects of A. japonica and A. anguilla were not very good by Alcian blue Safranin O staining, and the mast cells could not be examined in A. japonica and A. anguilla by the modified toluidine blue staining. It was firstly demonstrated that Nile tilapia mast cells contain tryptase in their cytoplasm by Elivision^TMplus immunohistochemical techniques with a murine monoclonal antibody AA1 raised against human mast cell tryptase. Small amount of the positive tryptase cells were found in Nile tilapia tissues and mostly lie in the base of mucosal epithelial cells and lamina propria in the sections of digestive tract. While the tryptasepositive mast cells had not been examined in A. japonica and A. anguilla. It showed that there was difference of biochemical element in cytoplasm granules among different fish mast cells. There were lots of tryptasepositive mast cells in the gastric carcinoma interstitium of human.