Platelets metabolize arachidonic acid to thromboxane A₂, a potent platelet aggregator and vasoconstrictor compound. The first step of this transformation is catalyzed by prostaglandin (PG) G/H synthase, a target site for nonsteroidal antiinflammatory drugs. We have isolated the cDNA for both human platelet and human erythroleukemia cell PGG/H synthase using the polymerase chain reaction and conventional screening procedures. The cDNA encoding the full‐length protein was expressed in COS‐M6 cells. Microsomal fractions from transfected cells produced prostaglandin endoperoxide‐derived products which were inhibited by indomethacin and aspirin. Mutagenesis of the serine residue at position 529, the putative aspirin acetylation site, to an asparagine reduced cyclooxygenase activity to barely detectable levels, an effect observed previously with the expressed sheep vesicular gland enzyme. Platelet‐derived growth factor and phorbol ester differentially regulated the expression of PGG/H synthase mRNA levels in the megakaryocytic/platelet‐like HEL cell line. The PGG/H synthase gene was assigned to chromosome 9 by analysis of a human‐hamster somatic hybrid DNA panel. The availability of platelet PGG/H synthase cDNA should enhance our understanding of the important structure/function domains of this protein and its gene regulation.—Funk, C. D.; Funk, L. B.; Kennedy, M. E.; Pong, A. S.; FitzGerald, G. A. Human platelet/erythroleukemia cell prostaglandin G/H synthase: cDNA cloning, expression, and gene chromosomal assignment. FASEB J. 5: 2304–2312; 1991.