Microsomal prostanoid synthesis was compared in normal gallbladders removed during organ donation and inflamed gallbladders removed at cholecystectomy. Normal human gallbladder microsomes demonstrated low rates of conversion of [14C]arachidonic acid to total labeled prostanoids, which increased during 1 to 30 min of incubation. Normal human gallbladder microsomes converted labeled substrate to all primary prostaglandins without demonstration of a major product. Inflamed human gallbladder microsomes increased the rate of conversion of [14C]arachidonic acid to total labeled prostanoids two or three times over the levels demonstrated by normal gallbladder microsomes at all times of incubation (p < 0.01). The main prostanoids synthesized by the inflamed human gallbladder microsomes were prostaglandin E2 and 6-keto-prostaglandin F1 alpha, which were increased four times over the levels demonstrated by normal gallbladder microsomes (p < 0.01). These data showed that inflammation of the human gallbladder was associated with increased synthesis of gallbladder 6-keto-prostaglandin F1 alpha and prostaglandin E2.