13 ,14 -Dihydro-15-keto-PGF2 alpha (PGFM) serum levels were determined by radioimmunoassay in 101 postpartum women who were treated with 200 micrograms methergin, 5 I.U. oxytocin and 500 micrograms sulprostone, respectively, 30 min after expulsion of placenta. All patients had normal deliveries. The present radioimmunoassay system did not show cross-reactivity with sulprostone. In addition, radioimmunoassayable sulprostone serum levels were monitored. Covariance analysis of area under PGFM serum levels between time zero and 180 min after application of oxytocics was performed. A higher but statistically not significantly PGFM serum level was maintained in subjects treated with sulprostone. Sulprostone serum levels are rapidly attained after application. Decrease of radioimmunoassayable sulprostone indicates a half-life of 75 min. These data corroborate clinical findings of an accompanying paper and combine to suggest that sulprostone may be a useful alternative therapy in high-risk patients with severe postpartum atony and hemorrhage in whom prior preventive measures have failed.