An observer-blind, single-centre study in which 7 groups of 25-30 male and female adults ≥18-50 years of age will be randomly allocated to receive two doses of cell culture non-adjuvanted or MF59-adjuvanted influenza A/California/4/2009 (H1N1) surface antigen vaccine containing 3.75, 7.5 or 15µg haemagglutinin by intramuscular injection. A second dose of the same vaccine containing the same quantity of antigen as in the first dose will be administered 21 days later. Three additional groups will receive two doses of MF59-adjuvanted influenza A/California/4/2009 (H1N1) surface antigen vaccine containing 7.5 µg haemagglutinin by intramuscular injection on day 0 or separated by 7 or 14 days. Subjects will be observed for local and systemic reactions for 30 minutes after each immunisation and will be monitored for any reactions and other adverse events for 7 days after each immunisation.