The hypothesis being tested is whether co-administration of yellow fever antibody and yellow fever vaccine (passive-active immunization) results in effective immunization while reducing viremia. The effectiveness of use of Gama STAN S/D® in combination with YF-VAX® will be assessed by a reduction in the viremic response in the vaccinee. Viremia is a measure of replication of YF 17D virus in host tissues, and absence of viremia indicates that virus replication in host tissues is abrogated. Abrogation of virus replication is expected to limit the potential for dissemination of virus and virus invasion of and damage to vital organs, including the liver and brain. The methods used to quantify viremia will be reverse transcriptase-polymerase chain reaction (RT-PCR) and direct plaquing in Vero cell monolayers. In addition, sera collected pre-vaccination and on blood draws post-vaccination will be tested for yellow fever neutralizing antibodies to measure immunogenicity. The method used to detect neutralizing antibodies will be a plaque-reduction neutralization test (PRNT) performed in Vero cell culture using the constant serum-varying virus technique standardized by the U.S. Food and Drug Administration (Beaty et al., 1989). The PRNT has been recently validated and used to support Phase 3 clinical trials of YF-VAX® (Monath et al. 2002a). The primary endpoint will be the comparison of the proportion of subjects without prior yellow fever immunization in each treatment group who have measurable vaccine virus after vaccination. The T-cell activation, cytokine response, dendritic cell response and kinetics and phenotype of natural killer (NK) cells will also be characterized.