Immunization with a synthetic peptide which is representative of part of the repeating region of Plasmodium falciparum circumsporozoite protein resulted in an immunity which allowed vaccinees to retard the development of patent malaria as compared to nonimmunized controls. Analysis of infection dynamics showed that immunity could be attributed to either neutralization of about 92% of inoculated sporozoites, delayed development of the majority of parasites, or a combination of neutralization and delayed development. In spite of this impressive antiplasmodial capacity, all volunteers after being bitten by infected mosquitoes developed malaria, and seven of eight developed parasitemia between 6.5 and 7.0 days after infective mosquito bites.