This report is concerned with clinical studies on a new method of chromo-endoscopy with Brilliant Blue (Food Blue No.1) (abbreviated as BB). In the indirect method, the pretreatment consists of an intramuscular injection of 4mg of Butropium bromide, followed by the patient swallowing a solution of BB and then a second injection of Butropium, after which an endoscopic examination is performed. In the direct method, the pretreatment consists of an intramuscular injection of Butropium followed by the patient swallowing a pretreatment solution without BB. In the latter method, a 0.05% BB solution is sprayed over the gastric mucosa through a cannula during the endoscopic observation. The following conclusions were obtained. Most of the cancerous lesions of the mucosal and submucosal type appeared reddened, whereas most of the bases of benign ulcers and erosions appeared blue. Thus, this method proved to be useful in the differential diagnosis of gastric cancer and benign gastric ulcer or erosion. In addition, its usefulness was enhanced by such features as brightness of viewing, good visualization of the mucosal surface through the dye solution and excellent quality of photographs. The indirect method proved to be superior to the direct method. The optimum concentration of BB dye in the indirect method was 100mg per 120ml for the pretreatment solution. BB dye appeared green in solutions containing more than 70mEq/l of hydrogen ions, both in vitro and in vivo. It appeared green in stomachs with maximal acidity of 98±27mEq/l (n=25), and appeared blue in those with maximal acidity of 33±28mEq/l (n=13) (p<0.005). The above-mentioned amount of BB dye used for this procedure is within the acceptable daily intake determined by WHO in 1969 for BB as food colour.