A functionally active human plasmin light (B) chain derivative, stabilized by the streptomyces plasmin inhibitor leupeptin, was isolated from a partially reduced and alkylated enzyme preparation by an affinity chromatography method with a L-lysine-substituted Sepharose column. This light (B) chain derivative was found to be relatively homogeneous by electrophoretic analysis in both an acrylamide gel/dodecyl sulfate system and on cellulose acetate. It possessed approximately 3% of the proteolytic activity (casein substrate) of the original enzyme, and it incorporated 0.09 mol of [3H]diisopropyl phosphorofluoridate per mol of protein. It contained 3.1 +/- 0.3 carboxymethylated cysteines per mol of protein and can be designated as a CmCys5-light (B) chain (CmCys)3. When this isolated light (B) chain derivative was mixed in equal molar amounts with streptokinase, the mixture developed both human and bovine plasminogen activator activities; the bovine activator activity was approximately 66% of the bovine activator activity of the equimolar human plasmin-streptokinase complex. Although this complex now incorporated 0.50 mol of [3H]diisopropyl phosphorofluoridate per mol of protein, its proteolytic activity, on a molar basis, was the same as the proteolytic activity of the isolated light (B) chain derivative. It was shown by electrophoretic analysis in both an acrylamide gel/epsilon-aminocaproic acid system and on cellulose acetate that the light (B) chain derivative and streptokinase forms an equimolar light (B) chain-streptokinase complex, indicating that the binding site for streptokinase is located on the light (B) chain of the enzyme. A functionally active equimolar light (B) chain-streptokinase complex was also isolated from a partially reduced and alkylated equimolar human plasmin-streptokinase complex by the affinity chromatography method. The plasminogen activator activities (human and bovine) of this light (B) chain-streptokinase complex were similar to those of the plasmin-streptokinase complex from which it was derived. Although this complex incorporated 0.70 mol of [3H]diisopropyl phosphorofluoridate per mol of protein, its proteolytic activity, on a molar basis, was only 14% of proteolytic activity of the plasmin-streptokinase complex.