The 15-methyl analog of prostaglandin F2 alpha (15-ME-PGF2 alpha), administered in a 3-mg dose via a single vaginal suppository and supplemented at 24 hours by intramuscular injection(s) of 250 micrograms, successfully induced abortion in 80 of 81 patients in the midtrimester of pregnancy. The mean abortion time was 19.6 hours. Two thirds of the patients aborted after treatment with the suppository alone in a mean time of 14.6 hours; the remaining 27 patients required intramuscular injections of 15-ME-PGF2 alpha to effect expulsion of the products of conception. Twenty-six of these 27 patients subsequently aborted in a mean total abortion time of 29.6 hours. Fifty-eight patients aborted within 24 hours of the initial prostaglandin administration, and 78 aborted by 36 hours. Parity and length of gestation did not significantly affect abortion time in this series, although the mean abortion time for parous patients and patients with gestations earlier than 17 weeks tended to be somewhat shorter than that of nulliparous patients and those with more advanced gestations. The placenta was spontaneously expelled in the majority of patients. Abortion was incomplete in 3 patients and required curettage. Uterine activity, as measured via an intraamniotic catheter in 6 patients, developed very gradually with the suppository, peaking at 3 hours after insertion, and was characterized by regular contractions with low intrauterine baseline tonus. The gastrointestinal side effects that occurred in 59% of patients who received the suppository were also most frequently observed at 3 hours after administration. In contrast the gastrointestinal disturbances elicited by intramuscular injections of the analog immediately followed the administration.