Patients with gastric cancer today coming under the care of surgeon often considerably malnourished. Because of a high incidence of postoperative complications and deaths in these patients, a study was instituted to investigate the state of their plasma proteins. This paper deals with the plasma protein concentrations in patients with gastic cancer, the effect of operation on them and the significance of plasma protein determination in therapy and as a prognostic aid. The concentrations of plasma protein in 43 patients with gastric cancer were pre- and post-operatively measured by copper sulfate specific gravity method, and 10 normal persons and 26 patients with gastroduodenal ulcer were studied as the control. The results obtained were as follows: 1. A significant decrease in the preoperative concentrations of plasma protein was observed in patients with gastric cancer. 2. Whether radical operation could be performed or not, and whether the gastric passageway was patent or not, made no difference in the plasma protein levels of patients with gastric cancer. 3. The determination of plasma protein was of little significance in the differential diagnosis between gastric cancer and gastroduodenal ulcer except in special cases. In the absence of massive hemorrhage or severe obstruction, plasma protein concentrations below 6.5 g/dl had some significance, those below 6.0 g/dl having a important significance. 4. The lowest level of plasma protein concentration in the patients who overcame a partial gastrectomy for gastric cancer was 4.72 g/dl. 5. Following resection of the stomach for gastric cancer, it was in most cases found that a decrease in plasma protein concentration occurred in the patients having the preoperative level abov 6.5 g/dl, whereas an increase occurred in those showing a level lower than 6.5 g/dl before operation. 6. The most pronounced drop in plasma protein concentration occurred about 7 to 12 days after gastric resection for cancer of the stomach. 7. Most of the patients with gastric cancer who underwent merely exploratory laparotomy or gastrojejunostomy because radical operation was impossible to be performed, showed a increased concentration of plasma protein after operation. 8. Death following operation for gastric cancer was found to occur in the patients who had low levels of plasma protein concentration.