Cytophotometric quantitative studies were made on the concentration and total amount of protein sylfhydrile groups in the gastric mucosa in different forms of cancer (30) and ulcerous disease (26) on resected stomach specimens. The analysis has demonstrated that the epithelial cells both in the ulcer zone and in cancerous tumor show the increased concentration and total amount of SH-groups. The histograms obtained indicated that in malignant tumor cells there occurs a shift of "the baseline" one class to the right. Changes in the content of SH-groups in the cells is likely to be one of the conditions for metabolic disturbances responsible for alterations in the division of cells and their carcinogenic transformation.