Peroral staining with tolonium chloride (toluidine blue) was performed in 45 patients with suspected gastric ulcer disease. During endoscopy, 19 of 21 malignant ulcers and one of 15 benign ulcers were stained. Following surgery, 18 of 21 malignant ulcers found in the surgical specimens were stained. Eleven patients with benign ulcers underwent surgery and none of these ulcers were found to be stained in the surgical specimens. Normal gastric mucosa and areas of gastritis appeared unchanged. The data suggest that tolonium chloride staining prior to endoscopy or surgery seems to be helful in differentiating between minute benign and malignant gastric ulcers.