The promoting effects of matrix metalloproteinase-2 (MMP-2) on lung metastasis of human fibrosarcoma cells (HT1080) were studied using nude mice. The fourth generation of HT1080 was established by consecutive clonal selection of metastatic lung nodules formed by intravenous transplantation. MMP-2 and MMP-9 in the culture supernatants of the first and fourth generation cells were analyzed by gelatin zymography and Western blotting, and quantified by scanning densitometry. In gelatin zymograms, mean ratios of values for the 59-kDa band (the active form of MMP-2) to those for the 72-kDa band (the inactive form of MMP-2) for optical density; area, and volume measured by densitometry were 1.44 +/- 0.12, 0.93 +/- 0.05, and 1.27 +/- 0.20, respectively, in the culture supernatant of fourth generation cells isolated from metastatic lung nodules. These values were significantly (P < 0.01) higher than those of first generation cells (0.70 +/- 0.04, 0.48 +/- 0.01, and 0.57 +/- 0.42). Three weeks after intravenous transplantation of HT1080 cells into nude mice, the incidence of lung metastasis and mean number and diameter of metastatic nodules formed by injection of first generation cells were 20% (2 of 10 mice), 2.9 +/- 0.2 and 2.0 +/- 0.2 mm, respectively; while they were 100%, 99.8 +/- 7.2 and 4.3 +/- 0.3 mm following injection of fourth generation cells. These findings suggest that the active MMP-2 produced by human fibrosarcoma cells is important for the cells to form lung metastatic lesions in nude mice.