Bicyclo Prostaglandin E1 (bicyclo PGE1) is a stable, base-catalyzed transformation product of the PGE1 metabolite 13,14-dihydro-15-keto PGE1. Bicyclo PGE1 is a stable, base-catalyzed transformation product of the PGE1 metabolite 13,14-dihydro-15-keto PGE1. 13,14-dihydro-15-keto PGE1 itself is a metabolite of PGE1 found in human plasma at a median level of 10 pg/ml. Due to the inherent instability of 13,14-dihydro-15-keto PGE1, it is advisable to quantify it as bicyclo PGE1 to estimate PGE1 biosynthesis or metabolism in vivo.