The involvement of tumor necrosis factor alpha (TNF alpha) in the regulation of acute-phase protein synthesis is currently under discussion. In this study the effect of human recombinant TNF alpha on the regulation of the 4 acute-phase proteins alpha 2-macroglobulin, albumin, alpha 1-proteinase inhibitor and alpha 1-acute-phase globulin was investigated in rat hepatocyte primary cultures. No changes in synthesis of any of the 4 proteins were observed. However, an acute-phase response similar to that in vivo could be generated by conditioned media from human monocytes containing natural TNF alpha. This response remained unchanged after neutralizing TNF alpha activity by the addition of a specific antibody to TNF alpha. It is concluded that the hepatocyte-stimulating activity synthesized by human monocytes is different from TNF alpha.