 Participants are randomly assigned to receive three injections of either the experimental malaria vaccine or a placebo a solution that does not contain the vaccine over a 2-month period. The shots are given in an upper arm muscle, each 1 month apart. On the day of each injection, participants give a history of symptoms since the last visit, have a brief physical examination and blood test and, for women, a blood or urine pregnancy test. After the injection, participants remain in the clinic 60 minutes for observation. In addition to the injections, participants undergo the following procedures: 