A randomized, double-blind, controlled clinical trial was conducted to evaluate the efficacy and safety of CJ-50300, a newly developed cell culture-derived smallpox vaccine, and to determine its minimum effective dose. The overall rates of cutaneous "take" reaction and humoral and cellular immunogenicity in CJ-50300 vaccinees were 100% (123/123), 99.2% (122/123), and 90.8% (109/120), respectively, and these rates did not differ significantly between the conventional-dose and the low-dose CJ-50300 (1.0x10(8) and 1.0x10(7) plaque-forming units/mL, respectively) (P>0.05 for each). No serious adverse reaction was observed. However, one case of possible generalized vaccinia occurred in the conventionally dosed group [ClinicalTrials.gov Identifier: NCT00607243].