A BLU:Ha newborn mouse lung adenoma bioassay was employed to compare the tumorigenicity of selected mononitroarenes and unsubstituted parent compounds 6 months after initial treatment. The presence of a nitro group had a variable effect upon compound potency in which tumorigenicity was increased, abolished, or unchanged. On the basis of results with equimolar doses, the potency of benzo[a]pyrene was greater than 6-nitrobenzo[a]pyrene (inactive), 6-nitrochrysene was much greater than chrysene (inactive), 3-nitrofluoranthene (active) was equal to fluoranthene (active), and 1-nitropyrene (inactive) was equivalent to pyrene (inactive). The potency series among the mononitroarenes was 6-nitrochyrsene much greater than 3-nitrofluoranthene greater than 6-nitrobenzo[a]pyrene (inactive) = 1-nitropyrene (inactive). Lung tumor incidence and multiplicity were similar for both males and females. No consistent pattern was observed for the occasional appearance of lymphoma or hepatic nodular hyperplasia in the various treatment groups.