The emergence of smallpox as a potential agent of bioterrorism has heightened concern about the vulnerability of the population to infection with this agent, and has led to proposals to undertake large scale smallpox immunization of military personnel and "first responders" in the United States, including certain health care workers. A particularly promising vaccine approach to the development of an effective, yet less reactogenic vaccine to smallpox is the use of Modified Vaccinia Ankara (MVA) as a vaccine. Despite the established efficacy of smallpox vaccination, the parameters of protective immunity against smallpox infection are incompletely understood. This is a phase I/II trial to be conducted under a placebo controlled double-blind, randomized allocation of study product. The purpose of this study is to assess the safety and immunogenicity of ACAM 3000 MVA in healthy vaccinia-naïve adult subjects. Participants will include 72 healthy, male or female, from the Boston metropolitan area. Six dose regimens will be studied initially: 10^6 or 10^7 tissue culture infectious dose 50 (TCID50) administered intradermally and 10^7 or 10^8 TCID50 administered intramuscularly or subcutaneously as 2 immunizations 1 month apart. Each arm will be comprised of 12 subjects, 10 of whom will receive ACAM3000 MVA and 2 of whom will receive placebo. A subsequent vaccinia vaccination will be offered to all patients. Consenting participants in Groups A and B will receive the vaccinia (Dryvax®) inoculation between 6 and 15 months after the initial MVA / placebo vaccination. Consenting participants in groups C, D, E and F will receive the Dryvax® inoculation approximately 6 months after the initial MVA / placebo vaccination, coinciding with planned visit 13. Assessment of safety will be carried out by observation and measurement of acute clinical and laboratory evidence of reactions or toxicity; including clinical, electrocardiographic or laboratory evidence of myopericarditis. Assessment of immunogenicity will be carried out by the measurement of humoral and cell-mediated immune response to ACAM 3000 MVA and vaccinia, performed on blood samples obtained at various times prior to and after immunization over the one year period of the study. Response to vaccinia will be assessed clinically (effect on a "take") and the results will be correlated with immune responses to MVA. Subjects will be followed for reactogenicity. Clinical assessments and blood samples will be obtained sequentially for immunogenicity determinations.