The effect of the skin tumor promoter benzoyl peroxide on the invasive potential of a murine squamous carcinoma cell line was investigated using an in vitro assay based on the capacity of cells to migrate through a porous filter coated with matrigel. Pre-treatment of the murine squamous carcinoma cell line CH72 with benzoyl peroxide added to the tissue culture medium increased the invasive capacity of these cells 5-8 times. No effects were observed on cells from primary cultures of normal murine epidermis. The simultaneous treatment of CH72 cells with benzoyl peroxide and the free radical scavengers, superoxide dismutase or CuSO4, prevented the increase and the values did not differ significantly from the baseline invasive potential. This suggests that activated oxygen species participate in the benzoyl peroxide-induced enhancement of the invasive capacity of these carcinoma cells.