An insoluble fibrinolytic enzyme with a molecular weight of approximately 30,000, was purified from the human spleen. A single protein band possessing fibrinolytic activity was obtained on polyacrylamide gel disk electrophoresis at pH 4.5. The enzyme, tentatively termed spleen fibrinolytic proteinase (SFP), degraded fibrinogen at neutral pH following Michaelis-Menten kinetics. The fibrinogenolytic activity was not inhibited by t-AMCHA, a specific plasmin inhibitor. SFP barely degraded certain synthetic ester or polypeptide substrates for trypsin, chymotrypsin, plasma, Xa, elastase and collagenase. These results indicate a different nature for SFP compared to other enzymes examined. SFP was found to digest no elastin and its fibrinogenolytic activity was strongly inhibited by STI, indicating that it was not an elastase. SFP required neither Zn++ nor CA++ for its fibrinogenolytic activity, indicating that it differed from metal-dependent proteinases such as collagenase. SFP was inhibited by DFP but not by TLCK, suggesting that it contains an active serine residue, but no trypsin type histidine at its active center. These results appear to show that SFP is a unique proteinase in the spleen, which is capable of degrading fibrin and fibrinogen at neutral pH.