1). Fluctuation of mast cells in the patients with gastric cancer, peptic ulcer and other surgical diseases was estimated by use of Arase's staining individually. It was found that mast cells were increased in cancer patients compared with the other cases. 2). They were remarkably increased in sarcoma and stomach cancer, successively in rectal and breast cancers. 3). No relation was observed in mast cells and severity of cancer. 4). Following removal of cancer they were decreased or become to normal, bnt increased in the non-resectable cases. 5). No relation was observed between the increasing rate of mast cells and white blood cells. 6). They were markedly decreased in the acute inflammatory diseases and tended to increase with its convalescence.