The author has observed the distribution of the tissue mast cells in 67 various tumors and precancerous lesions which occurred in the oral cavity. The specimcns ware obtained from the department of oral pathology, college of dentistry, Seoul National University, from Jan. 1970 to June, 1973. The results are as follows: 1) The number of the tissue mast cell was decrease predominantly in malignant tumors, especially in squamous cell carcinomas and in sarcomas. 2) The number of the tissue mast cell distirbution in adenocarcinomas one of malignant group was sligtly increased in with healthy oral mucosa. 3) The number of tissue mast cells in ameloblastomas one of benign group of the tumor of epithelial originwas more decreased than that in healthy oral mucosa. 4) The number of tissue mast cells in fibromas was more than that in healthy oral mucosa. 5) The number of the tissue mast cells in mixed tumors was increased one and a half times as many as that in healthy oral mucosa. 6) The number of the tissue mast cells in mixed tumors was increased one and a half times as many as that in healthy oral mucosa. 7) The tissue mast cell distribution can be observed more densly in the stroma of tumors than in the parenchyme of tumors.