The circumsporozoite (CS) protein of Plasmodium vivax consists of a central repeat region flanked by highly conserved non-repeat regions. Serum samples from 33 individuals with naturally acquired infections of P. vivax were tested for antibodies to four antigens representing the vivax CS protein. Three recombinant proteins containing different overlapping sequences in the non-repeat regions and either the entire central repeat region (vivax-1 and vivax-2) or two of the repeat sequences (vivax-3) were used as antigens in an enzyme-linked immunosorbent assay (ELISA). Antibodies to two other proteins, one (NS1(81)V20) containing the entire predominant repeat region (GDRAA/DGQPA) and the other (Pvk247) containing the variant repeat sequence (ANGAGNQPG) that was recently reported from Thailand were also measured by ELISA. Immunoglobulin G antibodies to the antigen representing the predominant repeat were present in 15% of the patients on the first day of treatment (day 0) and in 24% of the patients two weeks later (post-treatment). Six and 12% of the patients had IgG antibodies to the antigen containing the variant repeat on day 0 and post-treatment, respectively. A larger proportion of the sera had antibodies to the three antigens containing the non-repeat sequences; on the first day of treatment and two weeks later, 79 and 97% of the patients, respectively, had antibodies to vivax-1, vivax-2, and vivax-3. In this sample of Peruvians naturally infected with P. vivax, the most prevalent antibody responses were targeted to epitopes in the non-repeat region of the CS protein rather than to epitopes in the repeat region.(ABSTRACT TRUNCATED AT 250 WORDS)