The most abundant prostaglandin precursor is arachidonic acid (or its precursor, linoleic acid). The isolation and identification of prostaglandin compounds in a particular tissue, knowledge of the biologic properties of these compounds, and the use of readily available inhibitors of the prostaglandin pathway by aspirin-like drugs constitute the main steps in identifying the clinically significant arachidonic acid-prostaglandin systems. However, arachidonate-prostaglandin products are still difficult to measure clinically. The inhibitory effect of hydrocortisone on slow-reacting substance of anaphylaxis production can be partly reversed by arachidonic acid, which may explain aspirin- and indomethacin-sensitive asthma. Acetaminophen, when coadministered orally with aspirin and indomethacin, prevents gastric erosion. Long-acting prostaglandin analogues are currently used in obstetrics and are being tested in the treatment of thrombosis, inflammation, and excessive gastric acid secretion.