One of the consequences of neoplastic transformation is deregulation of tissue collagen metabolism. Although metalloproteinases initiate the breakdown of collagen in lung carcinoma, the final step of collagen degradation is mediated by prolidase (E.C.3.4.13.9). We investigated whether prolidase activity could reflect disturbances of collagen metabolism in human lung carcinoma planoepitheliale (Ca pl.). Ten human lung Ca pl. and 10 samples of normal lung parenchyma were compared with respect to prolidase activity and expression (western immunoblot), the content of collagen and collagen degradation products (free and bound hydroxyproline determination), beta1 integrin subunit expression (western immunoblot) and collagenolytic activity (zymography). An increase in collagen content (66%, P < 0.05), free proline pool (50%, P < 0.05) and collagenolytic activity was accompanied by a significant increase in the prolidase activity (106%, P < 0.05) and its expression in Ca pl. No differences were found between Ca pl. and the control lung tissue with respect to beta1 integrin expression. Prolidase activity may reflect disturbances in tissue collagen metabolism in lung Ca pl. and it may, therefore, serve as a sensitive marker of the disease.