A single intra-amniotic injection of (15S)-15 methyl prostaglandin F2α (THAM) was used to induce second trimester abortion in five patients. Serial levels of (15S)-15 methyl prostaglandin F2α were subsequently measured in amniotic fluid and plasma by radioimmunoassay. The slow removal of this drug from the amniotic fluid was documented. Plasma levels of (15S)-15 methyl prostaglandin F2α increased fourfold to sevenfold after clinical rupture of the membranes in three patients, supporting the fact that prostaglandins are well absorbed from the vagina. Because this analogue of prostaglandin can cause marked peripheral bronchoconstriction when administered systemically, it is best to avoid its use in patients with a history of asthma.