Absolute peripheral blood monocyte counts and the β-galactosidase activity of monocytes were studied in 60 patients with lung cancer, 20 with malignant lymphoma, 17 with other malignant diseases, 13 with sarcoidosis and 28 normal individuals. The monocyte count was 296.3±176.8/cmm in lung cancer, 315.6±160.0/cmm in malignant lymphoma, 217.8±89.6/cmm in sarcoidosis, and 298.6±144.1/cmm in normal individuals. Although there was no significant differences in the monocyte count of malignant and non-malignant diseases, all patients with lung cancer or malignant lymphoma had monocyte counts less than 100/cmm. In patients with lung cancer and malignant lymphoma, there was no significant differences in histological types or clinical stages. The monocyte count in lung cancer patients treated with streptococcal agent OK-432 was elevated to 484.9±269.0/cmm, as compared with 315.5±107.7/cmm before administration. A nadir of monocyte counts was found at one week after combination chemotherapy but recovery to normal value was observed by 3 weeks after. On the other hand, white blood cell counts reached a nadir 2 weeks after chemotherapy and recovered by 4 weeks after. The percentages of β-galactosidase positive monocytes were 21.5±11.5% in patients with lung cancer, 20.1±7.5% in malignant lymphomas, 19.1±6.8% in other malignant diseases and 25.3±12.4% in normal individuals. There was no significant differences in β-galactosidase activity related to histological types or clinical stages of patients with lung cancer and malignant lymphoma. No changes in β-galactosidase activity were found between before and after combination chemotherapy.