U. urealyticum, a member of the family Mycoplasmataceae, is often detected in the vagina of pregnant women. In this study, the possible association of ureaplasmal infection with preterm delivery was examined, as was the capacity of ureaplasmal LP to stimulate monocytes in vitro to produce pro-inflammatory cytokines relevant to preterm delivery. A hundred cases of normal delivery and 45 cases of preterm delivery were randomly selected. A mAb against U. urealyticum urease, that selectively and positively stained it in vaginal secretions of infected women but not in those of uninfected women, was generated. The preterm delivery group showed a significantly higher incidence of vaginal infection with this bacteria than the normal delivery group. Since the LP of Mycoplasma has potent biological activity, ureaplasmal LP was extracted. THP-1 cells, and human monocytic cells, produced IL-8, a potent pro-inflammatory cytokine associated with preterm delivery, and showed apoptotic cell death in response to the LP in vitro. These results suggest that U. urealyticum infection might play a causative role in preterm delivery via LP-induced IL-8 production and apoptosis.