Several human cell lines were studied for the production of gelatinases. Diploid fibroblasts, the melanoma cell line Bowes, the MG-63 osteosarcoma cell line and the human hepatoma cell line Malavu all constitutively produced a 67 kDa gelatinase. Gelatinolytic enzymes were quantified by a sensitive zymographic substrate conversion assay. Upon induction with phorbol 12-myristate 13-acetate (PMA), the human hepatoma cell line secreted considerable amounts of an 85 kDa gelatinase activity. The induction process was time- and dose-dependent. It represented a true increase in production per individual cell and was associated by a marked change of the cell morphology. The effect of various proteinase inhibitors and the maximal activity of the enzyme near neutral pH demonstrate that it is a neutral metalloproteinase. Characterization studies showed the 85 kDa gelatinase to be transformed to lower molecular weight, active forms by treatment with p-aminophenylmercuric acetate (APMA) or trypsin.