The purification of a 28 kDa serine protease from the body of a Korean polychaeta, Periserrula leucophryna is reported. This protease is active up to 50 degreesC, and over a wide pH range, between 4 and 12. The purified protease is resistant to detergent, retaining 50% activity even in the presence of 5% sodium dodecyl sulphate. It is not influenced by the presence of bleaching agents. Purification were achieved by adsorption with Diaion HP 20 (a styrene-divinylbenzene polymer), hydrophobic chromatography on a Phenyl-Sepharose and finally by affinity chromatography on a Benzamidine-Sepharose column. Adsorption with the adsorbent resulted in a recovery rate of 75% with a specific activity of 850 U mg(-1) protein and 49-fold of purification. Using the eluent fraction from the synthetic adsorbent as a starting material, the enzyme was finally purified 10 550-fold with a specific activity of 184 600 U mg(-1) protein. The use of an adsorption procedure as a first step proved to be a highly efficient way of purifying the protease from crude extracts of P. leucophryna.