Based on our previous observation that peritoneal endometriotic (PE) lesions synthesize in vivo substantially more haptoglobin (Hp) than related eutopic tissues, we hypothesized that this increase in Hp production was due to endometrial-peritoneal interactions. As interleukin-6 (IL-6) stimulates Hp in other tissues and is produced by endometrial cells, we tested our hypothesis by evaluating the effects of IL-6 on Hp production by PE cells, normal peritoneal (P) cells, and eutopic endometrial cells from women with (UE-E) and without endometriosis (UE-C) using semiquantitative RT-PCR and enzyme-linked immunoabsorbent assay. Endogenous production of IL-6 was also assessed. Treatment with human recombinant IL-6 and dexamethasone significantly increased Hp production by P or PE cells in a dose- and time-dependent manner (P < 0.05). Hp messenger ribonucleic acid was not detected in UE-E and UE-C cells in the absence or presence of IL-6 and dexamethasone. PE and UE-E cells expressed significantly more IL-6 messenger ribonucleic acid than P and UE-C cells (P < 0.05). Moreover, UE-E cells secreted 6 times more IL-6 protein than UE-C cells (P < 0.05). These findings support our hypothesis and suggest a novel endometrial-peritoneal interaction whereby locally synthesized IL-6 and Hp may participate in the establishment and persistence of peritoneal endometriosis.