To improve the effectiveness of 4-hydroxyphenylretinamide (4-HPR), an analogue of retinoic acid used in chemoprevention and treatment of breast cancer, we investigated the effect of concomitant administration of 4-HPR (0.1, 1 µM) and tamoxifen (TAM, 0.1, 1 µM), or 4-HPR and interferon-β (IFN-β, 10, 100, 500 IU/ml) on the growth offour cell lines (MCF7, T47D, MDA-MB231 and BT20) characterized by a different steroid receptor profile. A high concentration of 4-HPR caused a significant inhibitory effect not only on the estrogen receptor-positive cell lines (MCF7 and T47D), but also on one (BT20) of the two estrogen receptor-negative cell lines. IFN-β displayed a dose-dependent inhibitory effect in all cell lines, but it was most evident in MCF7 cells. In all cell lines, the combination of 4-HPR (0.1 µM) and TAM (1 µM) or IFN- β (500 IU/ml) generally caused additive or synergistic effects. In particular, the finding that in estrogen receptor-negative MDA-MB231 cells 4-HPR (which at 1 µM was singly ineffective) in combination with TAM at 1 µM or any concentration of IFN- β produced a synergistic effect suggests that the compound could act through a pathway independent of specific receptors for retinoids. Our results indicate that intrinsic characteristics of cells can influence responsiveness to 4-HPR, TAM and IFN- β, singly or in association, ever within cell lines with similar steroid receptor profiles. Thus, more attention should be payed to the biological characteristics of the single tumor in order to help choose the best combination of drugs to