An extracellular endo poly(beta-D-1,4-mannuronide) lyase of Dendryphiella salina IF 32139 was purified to homogeneity by Q Sepharose FF and Sephacryl S-200 HR column chromatographies. The purified enzyme had a molecular weight of 35,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and an isoelectric point of 3.65 by isoelectric focusing. The optimum pH and temperature for enzyme activity were pH 5.0 and 45 degrees C, respectively. The enzyme was stable from pH 4 to 10 and at temperature below 40 degrees C. Some divalent cations, Ca2+, Mn2+, and Zn2+, increased the enzyme activity. Hg2+ and NBS strongly inhibited the activity. This enzyme susceptibly degraded poly-M, produced a wide range of 4,5-unsaturated oligomannuronic acids, and further degraded these unsaturated oligomannuronic acids to produce the unsaturated monomer and dimer as final products.