Knowledge of the mechanisms of carcinogenesis is helpful for planning strategies and in the rational choice of agents for cancer prevention. There is a great potential for intervention at the promotion step of human carcinogenesis. ODC induction is associated with the promotion stage of carcinogenesis. Consequently, DFMO may be a useful drug for cancer prevention in humans. Long-term medication with higher doses (9 gm/m2/da) of DFMO has resulted in several toxic side effects, such as thrombocytopenia and reversible ototoxicity. However, doses of DFMO (less than 1 gm/m2/da), selected by our in vitro human skin punch biopsy assay (16, 47), may be given for a longer period without appreciable toxicity and should be evaluated in human cancer prevention trials.