In this study we examined the production of gelatinases A and B (MMP-2 and MMP-9), and their natural inhibitors TIMP-1 and TIMP-2 in cell lines derived from different histologic types of lung cancer. Gelatinolytic activity was measured by zymography and radiolabeled gelatin degradation. Immunocytochemistry and Western blot analysis were performed to corroborate the presence of immunoreactive MMP-2, MMP-9, TIMP-1 and TIMP-2 proteins. The highest gelatinolytic activity was identified in the cell extracts from a small-cell carcinoma cell line. MMP-9 was observed in all samples as a proenzyme, while MMP-2 was present as zymogen in the squamous-cell and in the small-cell carcinomas, and in its active form in one squamous-cell carcinoma cell line. TIMPs were also present in the neoplastic lung cell lines. TIMP-1 was observed in the media of all cells as a 21-kD band, and as TIMP-1 polymers with the exception of the small-cell carcinoma samples. TIMP-2 was found as higher-order molecular immunoreactive complexes that may correspond to proMMP-2/TIMP-2 complexes. These results demonstrate that lung neoplastic cells produce both MMP-2 and MMP-9 and their inhibitors, with the small-cell carcinoma cell extracts showing the highest enzymatic activity. This gelatinolytic activity fits well with the clinical metastatic behavior of this type of lung cancer.