A carrageenan-degrading marine Cellulophaga lytica strain N5-2 was isolated from the sediment of carrageenan production base. A -carrageenase (EC 3.2.1.83) with high activity was purified to electrophoretic homogeneity from the culture supernatant by a procedure of ammonium sulfate precipitation, dialyzing and gel filtration on SephadexG-200 and SephadexG-75. The purified enzyme was verified as a single protein on SDS-PAGE, and whose molecular weight was 40.8 kDa. The -carrageenase yielded a high activity of 1170 U/mg protein. For -carrageenase activity, the optimum temperature and pH were 35 degrees C and pH 7.0, respectively. The enzyme was stable at 40 degrees C for at least 2.5 h. The enzyme against -carrageenan gave a K-m value of 1.647 mg/mL and a V-max value of 8.7 mol/min/mg when the reaction was carried out at 35 degrees C and pH 7.0. The degradation products of the -carrageenase were analyzed by thin layer chromatography (TLC), high performance liquid chromatography (HPLC), electrospray ionization time-of-flight mass spectroscopy (ESI-TOF-MS) and C-13-NMR spectroscopy, and the results indicated that the enzyme was specific of the -1,4 linkage and hydrolyzed -carrageenan into -neocarraoctaose-sulfate and -neocarrahexaose-sulfate first, and then broke -neocarraoctaose-sulfate into -neocarrabiose-sulfate and -neocarrahexaose-sulfate.