The authors examined 40 patients with malignant tumors of various histogenesis, sites and extent, as well as 5 patients with benign tumors and other non-tumorous diseases. They also studied their electroencephalography and peripheral blood lymphocytic and erythrocytic ultrastructure in metabolic alkalosis temporarily induced by intravenous sodium hydrogen carbonate. In cancer patients without late metastases, alkalosis caused a transient normalization of previously altered electroencephalography, erythrocyte disaggregation and substantially reduced the count of killer cells in small and middle lymphocytes. These findings suggest that patients with malignant neoplasms have a generalized intracellular acidosis which can be temporarily abolished by plasma alkalinization.