5 years, a large number of these antigens have been cloned so that they may be investigated for their potential as components of a malaria vaccine. As a result of the sequencing studies on these cloned genes or gene fragments, a large amount of information has been obtained concerning the primary structural characteristics of Plasmodium fa/ciparum’ For several protein antigens that exist in antigenically distinct forms in different isolates of Pfalciparum, a number of different genes have been sequenced. A comparison of these sequences has revealed a remarkable degree of diversity among allelic genes for several different asexual blood stage antigens. As the asexual stages of plasmodia are haploid, the sequences of these allelic genes define the protein expressed in the different parasite isolates. One of these antigens is a heat-stable soluble antigen known as the S-antigen that can be detected in the serum of infected individuals. Two other highly polymorphic antigens are integral membrane proteins associated with the surface of the merozoite, the life cycle stage that is released from rupturing schizont-infected erythrocytes and subsequently invades another erythrocyte. The structural basis of diversity in these and other Pfalciparum antigens is described in this review. S-Antigens