Region II of the 175-kDa erythrocyte-binding antigen (EBA-175RII) of Plasmodium falciparum is functionally important in sialic acid-dependent erythrocyte invasion and is considered a prime target for an invasionblocking vaccine. The objectives of this study were to (i) determine the prevalence of anti-EBA-175RII antibodies in a naturally exposed population, (ii) determine whether naturally acquired antibodies have a functional role by inhibiting binding of EBA-175RII to erythrocytes, and (iii) determine whether antibodies against EBA-175RII correlate with immunity to clinical malaria. We treated 301 lifelong residents of an area of malaria holoendemicity in western Kenya for malaria, monitored them during a high-transmission season, and identified 33 individuals who were asymptomatic despite parasitemia (clinically immune). We also identified 50 clinically susceptible individuals to serve as controls. These 83 individuals were treated and monitored again during the subsequent low-transmission season. Anti-EBA-175RII antibodies were present in 98.7% of the individuals studied. The antibody levels were relatively stable between the beginning and end of the high-transmission season and correlated with the plasma EBA-175RII erythrocyte-binding-inhibitory activity. There was no difference in anti-EBA-175RII levels or plasma EBA-175RII erythrocyte-binding-inhibitory activity between clinically immune and clinically susceptible groups. However, these parameters were higher in nonparasitemic than in parasitemic individuals at enrollment. These results suggest that although antibodies against EBA-175RII may be effective in suppressing some of the wild parasite strains, EBA-175RII is unlikely to be effective as a monovalent vaccine against malaria, perhaps due to allelic heterogeneity and/or presence of sialic acid-independent strains.A vaccine against Plasmodium falciparum that can prevent the morbidity and mortality (8, 9, 25) from this parasite is greatly needed. Among the antigens being considered for vaccine development is the erythrocyte-binding antigen 175 . This is a 175-kDa protein that is expressed in the micronemes of merozoites (24), the stage of the parasite that invades erythrocytes. Its potential as a malaria vaccine antigen is based on the fact that the majority of P. falciparum isolates use EBA-175 as a ligand for the invasion of erythrocytes (6, 18). EBA-175 binds to sialic acid-dependent epitopes on erythrocyte glycophorin A (11, 23) and is probably involved in the formation of a junction between the erythrocyte and the apical portion of the merozoite just before invagination (10). This step is a key part of the erythrocyte invasion process and provides a logical target for vaccine-mediated immunity (23).EBA-175 is structurally divided into seven regions (1), and the cysteine-rich region II functions as the erythrocyte-binding ligand domain (23). Region II contains epitopes recognized by antibodies that block erythrocyte invasion (15,19,22) and by antibodies eluted from immune clusters of merozoites (21...