Midtrimester abortion was successfully induced in 29 of 30 patients with serial intramuscular injections of 15(S)-15-methyl-prostaglandin E2 methyl ester (15-ME-PGE2). The mean abortion time was 9.52 hours; parous patients aborted somewhat faster (mean, 8.76 hours) than nulliparous patients (mean, 10.47 hours). Eight patients were monitored throughout the abortion procedure and uterine activity was calculated and analyzed. Uterine response to a single injection of 5 mug 15-ME-PGE2 was characterized by the rapid appearance of low-amplitude, high-frequency contractions accompanied by a rise in intrauterine baseline tonus. Uterine activity rose to a mean of 500 Montevideo Units within 40 minutes of the initial intramuscular injection of 15-ME-PGE2. The most frequently encountered side effect of intramuscular injections of 15-ME-PGE2 was temperature elevation of 2 degrees F. or higher, which occurred in 29 of 30 patients. Five patients complained of shaking and chills but only five patients had any gastrointestinal side effects. From this study it appeared that on a weight-for-weight basis 15-ME-PGE2 is at least 20 times more potent than 15-ME-PGE2alpha and 1,000 time more potent than the naturally occurring PGE2.