Accumulation of ornithine decarboxylase (ODC) mRNA was investigated in human monocytes and mouse peritoneal macrophages. Treatment of both populations of mononuclear phagocytes with bacterial lipopolysaccharide induced a marked and rapid increase in the accumulation of the ODC gene transcript. A similar phenomenon, albeit less pronounced, was also observed following treatment of human monocytes with human recombinant interferon-gamma. These results suggest a role for ODC, and therefore polyamines, in the regulation of mononuclear phagocyte functions.