 The study seeks to define, with precision, the safety and efficacy of APSV, two dose potencies of one lot in vaccinia-naive adults. The ability of this vaccine to induce a classic take as defined by formation of a lesion at the injection site consistent with the Jennerian process see Appendix A and vaccine specific immune responses will be assessed. The local cutaneous lesion is considered to be a surrogate for clinical effectiveness of live, replicating smallpox vaccines. Neutralizing antibody formation has been correlated with takes for the Dryvax vaccine and is considered to be cross-protective for other orthopoxviruses, including variola. The study also seeks to characterize preliminarily the cell mediated immune CMI response to APSV as compared to Dryvax. 