We previously found that beta-carotene (betaCT) can act as a co-carcinogenic agent enhancing the cell transforming activity of powerful carcinogens such as benzo(a)pyrene (B(a)P) and cigarette-smoke condensate (TAR) in an in vitro medium-term ( approximately 8 weeks) experimental model utilizing BALB/c 3T3 cells (Mutat. Res. 440 (1999) 83-90). Here, we investigated whether vitamin E (VitE) and alpha-naphthoflavone (alphaNF) are able to affect the co-carcinogenic activity of betaCT in terms of inhibiting B(a)P and TAR cell transforming potential. The following experimental schedules were performed: (i) cultures treated for 72 h with chemicals in various experimental combinations (acute treatment); (ii) cultures grown in presence of tester agents for the whole period of the assay (chronic treatment) to more closely mimic human exposure. While the co-carcinogenic potential of betaCT was confirmed on both B(a)P and TAR, the latter being ineffective by itself, we found in repeated experiments that the presence of VitE or alphaNF significantly reduced the betaCT's enhancing effect in the formation of transformation foci by B(a)P and TAR. The mechanism of the inhibition could be explained by the known ability of alphaNF to inhibit cytochrome P450-linked B(a)P-bioactivating monooxygenases, while VitE may contrast the prooxidant activity of betaCT (e.g., oxygen radicals overgeneration). While highlighting the importance of increasing knowledge of the role of single provitamins, vitamins and micronutrients, our findings also underline the potential advantages of combining several dietary supplements in in vitro preventive investigations.