Interleukin-8 (IL-8) exerts unique chemotactic and activating activity on neutrophils. To address the significance of IL-8 in the fetoplacental unit during pregnancy, we cultured human placental explants that had been obtained by vaginal delivery, Caesarean section, or artificial abortion and then measured the IL-8 titer in the culture supernatants by enzyme immunoassay (EIA). Chorionic tissue from the first trimester produced a significant amount of IL-8 (2.2 +/- 0.4 ng/ml/10 mg, n = 5), while placentae in the second trimester (8.3 +/- 1.6 ng/ml/10 mg, n = 7) or at term (9.2 +/- 0.7 ng/ml/10 mg, n = 29) produced significantly higher amounts of IL-8. The presence or absence of labor did not affect the amount of placental IL-8 production. However, placentae with chorioamnionitis (25.2 +/- 1.6 ng/ml/10 mg, n = 9) showed significantly higher IL-8 production than those without chorioamnionitis (p less than 0.0001). Northern blot analysis of IL-8 mRNA expression demonstrated a constant level during pregnancy with or without chorioamnionitis, indicating the possibility that the major site of regulation of IL-8 synthesis in the placenta is posttranscriptional. Immunohistochemical analysis of first and third trimester placental tissues with rabbit anti-IL-8 antibody revealed the IL-8 producing cells to be trophoblasts and macrophage-like cells. IL-8 produced by the placental cells might contribute to potentiation of the immunocompetence of placental cells against bacteria invading the fetoplacental unit.