Cellular chromosomal damage is ubiquitously seen in HTLV-I-transformed lymphocytes. It is also characteristic of cells that have been exposed to mutagens. A sensitive measurement for mutagen-induced DNA damage is the formation of micronuclei in treated cells. Because current evidence suggests that HTLV-I Tax is etiologically linked to transformation, we tested for its activity in inducing micronuclei. We show here that transfection into cells of a Tax-producing plasmid rapidly induced the formation of micronuclei. This effect cooperated with that of a mutagen (mitomycin C) and was correlated with the inherent trans-activation capacity of Tax. These findings suggest that a commonly used mutagen assay could be a quick biological test for putatively oncogenic proteins.