 The purpose of this Phase 1 clinical trial is to evaluate the safety, reactogenicity and immunogenicity of two malaria transmission-blocking vaccines, PpPfs25 and ScPvs25, in healthy adult volunteers. Of the four species of malaria that infect humans, Plasmodium falciparum is responsible for the majority of these deaths. However, outside of Africa, most of the malaria is caused by Plasmodium vivax; although these cases result in few deaths, they represent a major cause of morbidity and lead to a significant impact on the quality of life. The development of a safe and effective vaccine that prevents the transmission of P. falciparum or P. vivax would be an important addition to the current methods for controlling the spread of malaria parasites. A vaccine designed to prevent oocyst development in the mosquito by eliciting transmission-blocking antibodies in the vertebrate host would protect other individuals in the vicinity from becoming infected from the immunized person. Thus, despite no immediate role in personal protection, transmission-blocking vaccines are a potentially powerful component of a multifaceted public health approach to controlling or eliminating malaria. This study will be conducted at the Johns Hopkins University - Bloomberg School of Public Health, Center for Immunization Research. Seventy-two healthy male and non-pregnant female volunteers ages 18-50 will be enrolled. This study will be single blinded to volunteers and placebo-controlled. The study will evaluate three dose levels of both the PpPfs25 and ScPvs25 malaria vaccines 5 microg, 20 microg, 80 microg emulsified with Montanide ISA51 as compared to Montanide ISA51 given alone. Seventy-two volunteers 60 vaccine and 12 placebo will be enrolled and assigned to one of six cohorts. In each cohort, 10 volunteers will receive vaccine and 2 will receive placebo. Volunteers will be vaccinated by intramuscular injection on days 0 and 120. The groups will be staggered such that adequate safety evaluation can be performed prior to dose escalation. The duration of the study is 18 months per volunteer. The primary objectives of this trial are to assess and compare, in a dose-escalating study, the safety and reactogenicity of the PpPfs25 and ScPvs25 transmission blocking vaccines as compared to adjuvant alone; and to assess and compare, in dose-escalating study, the duration of the antibody response over an 18 month period in each volunteer as well as the effect of boosting with a second vaccination at 4 months. 