Transforming growth factor beta (TGF beta) increased the arachidonate 5-lipoxygenase (5-LO; EC 1.13.11.34) activity in HL-60 cells induced to granulocytic differentiation by dimethyl sulfoxide. The presence of a factor in human serum that caused a similar increase was recently demonstrated. Several observations indicate that the serum factor consists of isoforms of TGF beta. Heat-treated serum and TGF beta both resulted in approximately 10-fold increased 5-LO activity of HL-60 cells, antiserum to TGF beta neutralized the 5-LO-increasing activity in serum, and physical properties of the serum factor (lipophilic nature, alkaline pI, stability to heat and acid) coincided with those of TGF beta. The pattern of activity of native and heat-treated sera is compatible with activation of a latent form of TGF beta in serum. This activity was specific for TGF beta, since none of several other cytokines could increase 5-LO activity in differentiating HL-60 cells. However, granulocyte/macrophage-colony-stimulating factor (GM-CSF) and tumor necrosis factor alpha enhanced the effect of TGF beta. The most prominent effects of TGF beta, whether alone or together with GM-CSF, were observed for 5-LO activity in intact cells (10-fold or 30-fold induction, respectively). 5-LO protein levels were less affected (up to 2- or 5-fold, respectively, as judged from Western blots). There was no appreciable effect of TGF beta, or a combination of TGF beta and GM-CSF, on 5-LO mRNA expression.