The leucine aminopeptidase activity has been determined by using the cytochemical method of Burston and Folk in peripheral blood neutrophils and lymphocytes of 45 patients with various malignancies. Lung cancer, carcinoma of the stomach and cancer of the colon was diagnosed in 24, 16, and 5 patients, respectively. Patients with metastases showed a significantly higher activity of the enzyme if compared with that in the control group of healthy subjects and patients without metastases. The percentage of enzyme-positive lymphocytes was elevated significantly in patients with metastases whereas a total percentage of lymphocytes with regard to differential leukocyte count was diminished both in patients with and without metastases. The absolute count of neutrophils was elevated both in patients with and without metastases. The authors discuss the significance of their observation with regard to the antitumor cytotoxic effect of neutrophils and lymphocytes.