Sodium diethyl dithiocarbamate (DTC) is a low molecular weight sulfur compound which has been previously shown to have immunomodulatory properties. In the present report, the effects of DTC on the mononuclear phagocytic system were studied in guinea pigs. The colloidal gold clearance was enhanced in animals which received an i.v. injection of 10 mg DTC on day – 13, but not in animals treated on day – 8 with DTC. DTC treatment modified neither 99mTc sulfur colloid uptake nor activation markers in peritoneal macrophage monolayers. These results show that DTC which modulates both delayed hypersensitivity and antibody synthesis only enhances nonspecific binding of macrophages but leaves unaffected endocytosis and metabolic status of macrophages.