Endometriosis, a frequent oestrogen-dependent disease believed to result from an aberrant proliferation of endometrial tissue outside the uterine cavity, is associated with an increased expression of monocyte chemotactic protein-1 (MCP-1) in the intrauterine endometrium. This makes it plausible that migrating endometrial cells are intrinsically able to initiate monocyte chemoattraction and activation, a phenomenon which has been consistently observed in the peritoneal cavity of patients and recently in their eutopic endometrium. To elucidate the mechanisms involved in the regulation of MCP-1 expression in eutopic endometrial cells, we studied the effects of ovarian hormones and found that oestradiol (10(-9) and 10(-8) mol/l) markedly increased MCP-1 mRNA steady-state levels and protein secretion by endometrial cells in response to interleukin-1beta (IL-1beta) (0.1 ng/ml). The IL-1beta-induced MCP-1 expression was even higher following pretreatment of cells with both oestradiol (10(-9) mol/l) and progesterone (5x10(-8) mol/l). This did not seem to be due to increased MCP-1 mRNA stability, but rather to a higher level of gene transcription. Our results provide evidence that ovarian steroids regulate, indirectly, the synthesis and the secretion of a potent chemotactic and activating factor for monocytes/macrophages by endometrial cells of women with endometriosis and reveal a new mechanism for oestradiol action.