Interferon-alpha (IFNalpha) can induce apoptosis, a process regulated by a complex network of cell factors. Among these, eukaryotic initiation factor-5A (eIF-5A) is peculiar because its activity is modulated by the post-translational formation of the amino acid hypusine. Here we report the effects of IFNalpha and epidermal growth factor (EGF) on apoptosis and eIF-5A activity in human epidermoid oropharyngeal KB and lung H1355 cancer cells. We found that 48-h exposure to 1000 and 2000 IU/ml IFNalpha induced about 50% growth inhibition and apoptosis in H1355 and KB cells, respectively, and the addition of EGF completely antagonized this effect. When IFNalpha induced apoptosis, a hyperactivation of MEK-1 and ERK signalling and a decrease of the hypusine-containing form and, thus, of eIF-5A activity were recorded. The latter effect was again antagonized by the addition of EGF to IFNalpha-pretreated cells, probably through the activation of the EGF-->ERK-dependent pathway, since the addition of the specific MEK-1 inhibitor PD098059 abrogated the recovery of intracellular hypusine content induced by EGF in IFNalpha-pretreated cancer cells. Subsequently, we evaluated if the hypusine synthesis inhibitor (and eIF-5A inactivator) N1-guanyl-1,7-diaminoheptane (GC7) synergized with IFNalpha in the induction of cell growth inhibition and apoptosis. The analysis of the isobologram of IFNalpha and GC7 demonstrated a strong synergism between the two drugs in inducing cell growth inhibition. We also found that GC7 and IFNalpha had a synergistic effect on apoptosis. These data suggest that the apoptosis induced by IFNalpha could be regulated by eIF-5A that, therefore, could represent a useful target for the potentiation of IFNalpha antitumor activity.