Prostacyclin synthase (PGIS) from bovine aorta was purified using conventional purification procedures including detergent solubilization, Sephacryl S-300 gel filtration and Mono Q high performance liqud chromatography. Polyclonal antiserum raised against the 52-kDa protein bound specifically to PGIS. After immobilization to Protein A-sepharose, the PGIS-antibody complex displayed PGIS activity. Results based on (1) elution profile of the Mono Q column, (2) two-dimensional gel electrophoresis and (3) N-terminal amino acid sequence suggested that PGIS is heterogenous. Amino acid sequences of N-terminus and a tryptic peptide led us to isolate a partial cDNA fragment.