Functions of peripheral blood monocytes in 13 patients with sarcoidosis were studied and compared to normal individuals. The mean absolute number of monocytes was 277±121/cmm(M±SD) in sarcoidosis patients and 299±144/cmm in controls. However, there was no significant difference between the two groups. Lysosomal enzyme activity of monocytes was histochemically determined by ability of staining to beta-galactosidase. The mean percentage of beta-galactosidase positive monocytes was 35.8±16.9 in sarcoidosis patients and 33.7±12.1 in controls. There was no significant difference between the two groups. Monocyte chemotactic response was measured in vitro by Boyden's millipore technique-using zymosan activated human serum as chemoattractant. Chemotactic response was significantly reduced in sarcoidosis patients(28.2±7.0) compared with controls (34.8±8.0). Ability of monocyte to phagocytize the complement coated zymosan was also significantly lower in sarcoidosis patients(49.2±24.1) compared with controls (84.1±9.4). Monocyte-mediated cytotoxicity was tested by the inhibition of (3)H-thymidine uptake by two tumor cell lines. The growth-inhibitory activity was 87.0±8.6 in sarcoidosis patients and 85.5±8.0 in controls using human squamous lung cancer cell line(EBC-1) for the target cell, and 47.5±14.0 and 46.4±11.8 respectively using Hella cell. There was no significant difference between the two groups using either target cell.