We have constructed a cDNA clone library that contains many natural immunogens of the asexual blood-stages of Plasmodium falciparum. The corresponding parasite antigens have been identified with antisera raised against the antigens expressed in Escherichia coli or with monospecific human antibodies purified on adsorbents prepared from various clones. Sequencing studies on the clones have revealed that many malaria antigens contain extensive sequence repeats. These repeats encode antigenic epitopes that in several proteins have been shown to be immunodominant. One candidate vaccine molecule, the Ring-infected Erythrocyte Surface Antigen (RESA), which is transferred from inside merozoites to the erythrocyte surface at about the time of merozoite invasion, contains two blocks of antigenically cross-reactive repeats. The structures of other antigens containing extensive repeats are described and their possible significance to the host-parasite relationship is discussed.