The pituitary hormone prolactin (PRL) is also produced extrapituitarily by cells of the immune system. In leukocytes PRL expression is directed by an alternative promoter, located 5800 bp upstream of the pituitary promoter. We have shown here that this alternative promoter is activated in myeloid leukemic cells by the proinflammatory cytokine tumor necrosis factor (TNF)-alpha. The effect of TNF-alpha on promoter activation was blocked by the protein kinase C (PKC)-inhibitor GFX109203. In addition, we have shown that the TNF-alpha-responsive region is located between -1842 and -1662 of the extrapituitary PRL promoter. Our findings might have clinical relevance in view of recent data indicating leukocyte-derived PRL is involved in auto-immune and hematological disorders.