A bacterium, Azotobacter chroococcum 4A1M, isolated from a soil sample, produced an alginate-decomposing enzyme in the culture broth. The enzyme was purified to an electrophoretically homogeneous state. The purified enzyme showed maximum activity at pH 6.0 and 60 degrees C; it was stable up to 60 degrees C at pH 6.0 and activated by Ca2+ and inhibited strongly by Hg2+. The molecular mass of the enzyme was estimated to be 23 kDa by sodium dodecyl sulfate/polyacrylamide gel electrophoresis and 24 kDa by gel filtration. Therefore, the enzyme was considered to be monomeric. The NH2-terminal amino acid sequence was determined to be H2N-Ala-Ser-Ile-Ala-Ile-Thr-Asn-Pro-Gly-Phe enzyme reacted only on the polymannuronate block of alginic acid, and two main reaction products were obtained when short-chain polymannuronate was used as a substrate. The degrees of polymerization of the two products were three and two respectively.