Nucleoli in lymphocytes were investigated in the peripheral blood of patients with gastric cancer and gastric ulcer. The number of "active lymphocytes" (lymphocytes with compact nucleoli or nucleoli with nucleolonemata as functionally dominant nucleoli) was increased in the peripheral blood of patients with gastric cancer as well as gastric ulcer. In contrast, the number of "resting lymphocytes" (lymphocytes with ring shaped nucleoli as functionally dominant nucleoli which under proper conditions can be stimulated with respect to the blastic transformation) was decreased in the peripheral blood of patients with gastric cancer. The decrease of these lymphocytes in number in the peripheral blood of patients suffering of gastric cancer might represent one of factors responsible for the spread of the malignant process.