Hepatitis C virus (HCV) cannot be grown in vitro, making biochemical identification of new drug targets especially important. HCV p7 is a small hydrophobic protein of unknown function, yet necessary for particle infectivity in related viruses Harada, T. et al., (2000) J. Virol. 74, 9498–9506. We show that p7 can be cross-linked in vivo as hexamers. Escherichia coli expressed p7 fusion proteins also form hexamers in vitro. These and HIS-tagged p7 function as calcium ion channels in black lipid membranes. This activity is abrogated by Amantadine, a compound that inhibits ion channels of influenza Hay, A.J. et al. (1985) EMBO J. 4, 3021–3024; Duff, K.C. and Ashley, R.H. (1992) Virology 190, 485–489 and has recently been shown to be active in combination with current HCV therapies.