Recently, a novel swine-origin influenza A/H1N1 virus was identified as a significant cause of febrile respiratory illnesses in Mexico and the United States. It rapidly spread to many countries around the world, prompting the World Health Organization (WHO) to declare a pandemic on June 11, 2009. Data from several cohorts in different age groups that received licensed trivalent seasonal influenza vaccines suggest that these vaccines are unlikely to provide protection against the new virus. In addition, adults are more likely to have measurable levels of serum hemagglutination inhibition assay (HAI) or neutralizing antibody than are children. These data indicate the need to develop vaccines against the new H1N1 strain and suggest that different vaccine strategies (e.g., number of doses, need for adjuvant) may be appropriate for persons in different age groups. The primary safety objective of this study is to assess the safety of the unadjuvanted, inactivated H1N1 vaccine when administered at the 15 or 30 microgram (mcg) dose. The primary immunogenicity objective is to assess the antibody response following a single dose of unadjuvanted, inactivated H1N1 vaccine, stratified by age of recipient, when administered at the 15 or 30 mcg dose. The secondary immunogenicity objective is to assess the antibody response following 2 doses of unadjuvanted, inactivated H1N1 vaccine, stratified by age of recipient, when administered at the 15 or 30 mcg dose. Participants will include up to 450 healthy adults age 18 and older who have no history of novel influenza H1N1 2009 infection or novel influenza H1N1 2009 vaccination. This is a randomized, double-blinded, Phase II study in healthy males and non-pregnant females designed to investigate the safety, reactogenicity, and immunogenicity of an inactivated influenza H1N1 virus vaccine at 2 dose levels. Subjects will be randomized into 2 dose groups, stratified by age (200 subjects per dose group with 100 subjects per age stratum, 18-64 or greater than or equal to 65 years of age) to receive intramuscular inactivated influenza H1N1 vaccine at 15 mcg (Group 1) or 30 mcg (Group 2). The H1N1 vaccine will be administered at Day 0 and Day 21. Following immunization, safety will be measured by assessment of adverse events through 21 days following the last vaccination (Day 42 for those receiving both doses and Day 21 for those who do not receive the second dose), serious adverse events and new-onset chronic medical conditions through 7 months post first vaccination (Day 201), and reactogenicity to the vaccine for 8 days following each vaccination (Day 0-7). Immunogenicity testing will include HAI and neutralizing antibody testing on serum obtained on the day of each vaccination (prior to vaccination), on Day 8-10 after each vaccination, and 21 days following the second vaccination (Day 42).