Four new isoforms of superoxide dismutase (SOD; superoxide: superoxide oxidoreductase, EC 220.127.116.11.) were identified in extracellular washing fluid from Scots pine (Pinus sylvestris L.) needles. The isoforms had an apparent molecular mass of 33 kDa. No neutral carbohydrates were present in the enzymes. The enzymatic activities were inhibited by 3 mM NaCN. One of the putative extracellular SOD isoforms was purified and NH2-terminal-sequenced. The sequence contained the domain KAVAVL. The domains VEG and V(K/S)G, present in chloroplastic and cytosolic CuZn SODs of plants, respectively, were not detected. The enzyme was composed of two subunits of 17.8 kDa each. The isoelectric point was determined to be 6.5. The results suggest the existence of an extracellular SOD in Scots pine.