Plasminogen activators produced locally in the skin have been implicated in blistering skin diseases. To explore whether plasminogen activators convert their substrate plasminogen into plasmin locally in the lesional skin we have analyzed the autoimmune blistering skin disease bullous pemphigoid. Enzyme activity was detected in bullous pemphigoid skin blister fluid by using a low molecular weight chromogenic substrate for plasmin. Enzyme activity was detected neither in suction blister fluid raised on normal skin nor in normal plasma. Immunoprecipitation or fractionation by molecular sieve chromatography of bullous pemphigoid skin blister fluid followed by testing in immunoassays disclosed putative plasmin/alpha 2-macroglobulin complexes and plasmin/alpha 2-antiplasmin complexes. Enzyme activity detected in bullous pemphigoid skin blister fluid by the low molecular weight chromogenic peptide assay was ascribed to the putative plasmin pha 2-macroglobulin complexes. Because formation of plasmin-inhibitor complexes requires the active plasmin, our findings indicate previous activation of plasminogen to plasmin in skin lesions. There was no evidence for free plasmin (i.e., plasmin not complexed to inhibitors) in bullous pemphigoid blister fluid, suction blister fluid, or plasma.