Our purpose was to investigate the effects of different classes of antibiotics, namely beta-lactamines, aminoglicosides, tetracyclines, macrolides, on amniotic prostaglandin E release to clarify their role in the treatment of premature labor. The effects of these antibiotics were tested also in combination with ampicillin, whose antiprostaglandinergic action had been demonstrated previously. Ceftriaxone and gentamicin significantly and reversibly inhibit both basal and arachidonic acid- or oxytocin-stimulated prostaglandin E release from amnion, although to a different extent. On the contrary, tetracycline and erythromycin do not influence prostaglandin E output. The inhibitory effect of ampicillin is potentiated, in an additive manner, by ceftriaxone, reduced by gentamycin, and eliminated by tetracycline and erythromycin. The finding that diverse classes of antibiotics and their combinations affect amniotic prostaglandin E release should be taken into account in the management of premature labor.