We identified and characterized type IV collagenase and gelatinase activity in pleural fluid from 32 patients. The capacity to substantially degrade type IV collagen was demonstrated in every pleural sample. Comparable results were also noted for the degradation of a radiolabeled gelatin substrate. Gelatin gel zymography of the pleural fluids revealed two prominent zones of lysis at 66 kDa and 92 kDa. These were identified by specific polyclonal antibodies as human matrix metalloproteinases MMP-2 and MMP-9. The concentration of MMP-2 in pleural fluid, as measured by enzyme-linked immunoassay, averaged 1,622 ng/ml whereas those of MMP-9 were 210 ng/ml. Substrate degradation activity was compared in both serum and pleural fluid from three patients and found to be similar. In serum this enzymatic activity was primarily due to MMP-9 whereas in pleural fluid, the predominant gelatinase was MMP-2. This was confirmed by immunoassay that showed that MMP-2 levels were two to five times higher in pleural fluid than in serum. We conclude that substantial amounts of MMP-2 and, to a lesser degree, MMP-9 are present in pleural effusions. The bioactivity and the immunoactivity of these enzymes did not help to distinguish among pleural fluids characterized as transudates, nonmalignant exudates, or malignant exudates. The differences in the distribution of these enzymes in pleural fluid and blood suggest that their presence is not due simply to the ultrafiltration of plasma, but rather to synthesis by the resident cells at the pleural surfaces.