Previously, we have reported the development of a new quantitative-competitive polymerase chain reaction (qc-PCR) method to evaluate interferon-beta (IFNbeta) bioavailability in multiple sclerosis (MS) patients, by measuring mRNA of mixovirus resistance protein A (MxA). Here we show that our assay is also able to assess IFNalpha bioavailability in hepatitis C virus (HCV) patients treated with different IFNalpha regimens. Indeed, our method was able to detect a slight constitutive expression of MxA mRNA in untreated HCV patients (median=70 fgMxA/pgGAPDH) and a significant induction 12 h after the first IFNalpha administration (median=750 fgMxA/pgGAPDH).