The isolation and cloning of a full-length cDNA insert complementary to mRNA encoding human aromatase system cytochrome P-450 is reported. The insert contains an open reading frame encoding a protein of 503 amino acids. This gene is clearly a member of the cytochrome P-450 gene superfamily, because the sequence contains regions of marked homology to those of other members, notably a putative membrane-spanning region, I helix, Ozols, and heme-binding regions. The cDNA was inserted into a modified pCMV vector and expressed in COS-1 monkey kidney tumor cells. The expressed protein was similar in size to human placental aromatase system cytochrome P-450, as detected by immunoblot analysis, and catalyzed the aromatization of androstenedione, testosterone, and 16 alpha-hydroxyandrostenedione. This activity was inhibited by the known aromatase inhibitors, 4-hydroxyandrostenedione and econazole. Thus the several steps involved in the aromatization reaction appear to be catalyzed by a single polypeptide chain, which can metabolize the three major physiological substrates.