Sodium salicylate inhibited generation of granulocyte and macrophage colonies when added to soft agar cultures of mouse or human bone marrow cells (CFUc) containing colony-stimulating factor (CSF). This effect was dose-dependent with over 90% inhibition at 48 mg%. The salicylate effect was not decreased with increasing concentrations of CSF, but inhibition was reversed when salicylate-treated CFUc were washed with drug-free medium before plating. CSF production was not inhibited by salicylate.Recently a number of drugs, phenobarbital (I), diphenylhydantoin (2), halothane and other anesthetics (3-5), salicylates (6-8), and oral contraceptives (9) have