The effect of Adriamycin on the invasive capacity of rat ascites hepatoma cells, W1, was studied. The invasive capacity of W1 cells was estimated in vitro by counting the number of penetrated single tumor cells and tumor cell colonies formed from the penetrated cells underneath a cultured mesothelial cell monolayer (H. Akedo et al., Cancer Res., 46: 2416-2422, 1986). A considerable increment of the invasive capacity was observed when the tumor cells had been treated with 1.0 to 20.0 microM Adriamycin. This augmentation of invasive capacity of tumor cells was partially inhibited by 60 microM N-acetylcysteine, a scavenger of free radicals. On the other hand, 60 microM N-acetylcysteine did not impair the cytotoxicity of Adriamycin for W1 cells measured by an in vitro tetrazolium-based colorimetric assay for cytotoxicity.