In 33 patients with lung cancer (6 women and 27 men, aged at average 61.2 years) the activity and intracellular localization of acid phosphatase, beta-glucuronidase and N-acetyl-beta-glucosaminidase in peripheral blood lymphocytes were determined by means of semiquantitative cytochemical methods. In comparison to the control group of healthy subjects, the patients with lung cancer showed increased counts of acid phosphatase-positive lymphocytes with granular-diffuse cytochemical reaction, increased counts of beta-glucuronidase-positive lymphocytes with solely granular type of reaction and increased numbers of N-acetyl-beta-glucosaminidase-positive cells showing the granular, granular-diffuse and diffuse type of reaction. The total count of beta-glucuronidase-positive and N-acetyl-beta-glucosaminidase-positive lymphocytes was significantly elevated in these patients. The authors discuss the significance of their observations for evaluating lymphocyte response in patients with lung cancer.