We have studied the cellular localization of the relaxin-like factor (RLF) in the histologically normal cyclic endometrium collected from days 3--26 of the menstrual cycle. RLF transcripts and protein were detected in the luminal and glandular epithelium and in stromal cells at all stages of the cyclic endometrium. Increased expression of RLF was observed in endometrial tissues in the proliferative as compared to the secretory phase, suggesting that oestrogens affect RLF gene activity in the human endometrium. The cellular localization of RLF transcripts and protein was also determined in first trimester placental tissues obtained from normal and ectopic tubal implantation sites and in third trimester placentae of normal and pre-eclamptic pregnancies. In first trimester placenta, weaker expression of RLF was observed in the syncytiotrophoblast as compared to the underlying cytotrophoblast. Extravillous trophoblast cells constitutively expressed RLF. Trophoblast cells were the main source of RLF in the human placenta and trophoblastic RLF gene activity was unaffected by either the site of implantation or the invasive properties of the cytotrophoblast as demonstrated by samples from patients with tubal implantation and pre-eclampsia respectively. Decidual cells weakly expressed RLF. The presence of unprocessed and cleaved immunoreactive RLF in term placenta was determined by Western analysis. The above results suggest a functional role for both RLF isoforms within normal placental tissue.