The cDNA for prostaglandin endoperoxide synthase (cyclooxygenase) was cloned from human platelets by the polymerase chain reaction amplification method, and the primary structure of the enzyme was deduced from the nucleotide sequence. The enzyme was composed of 599 amino acids including 23-amino acid signal sequence, and the calculated molecular weight of the mature protein was 65,995. The enzyme was immunoaffinity-purified from human platelets. The N-terminal amino acid sequence determined by Edman degradation was Ala-Asp-Pro-Gly-Ala-Pro-Thr-Pro-, and the result confirmed the primary structure of the enzyme, which was deduced from the cDNA sequence.