Hepatitis C virus (HCV) envelope glycoproteins E1 and E2 were used with MF59 adjuvant as a candidate vaccine for a phase 1 safety and immunogenicity trial. Ten of 41 vaccinee serum samples displayed a neutralization titer of > or =1:20 against vesicular stomatitis virus (VSV)-HCV pseudotype, 15 of 36 serum samples tested had a neutralization titer of > or =1:400 against human immunodeficiency virus (HIV)-HCV pseudotype, and 10 of 36 serum samples tested had a neutralization titer of > or =1:20 against cell culture-grown HCV genotype 1a. Neutralizing serum samples had increased affinity levels and displayed >2-fold higher specific activity levels to well-characterized epitopes on E1/E2, especially to the hypervariable region 1 of E2.