Abstract:Seaweed (Gracilaria gigas) is an edible red alga and occasionally induces food poisoning cases. Prostaglandin E2 (PGE2) has been reported to be possible causative agent. In this study, a simple, sensitive, rapid and accurate HPLC method was developed for quantifying prostaglandins in seaweed. The mobile phase was gradient acetonitrile (35–60%) and 0.017 M phosphoric acid at flow rate of 1 mL/min within 30 min. The standard curves of prostaglandins were extremely linear (R2 > 0.999) with low correlation coefficients (less than 4.7) in the range of 5–50 μg/mL. To obtain maximum prostaglandins amount, the optimal ratio of seaweed (wet weight) to arachidonic acid was 10 g:2 mg and oxygen was needed in reaction.