We studied seroreactivity against Plasmodium vivax antigens in 62 individuals living in a small community near Mantena, Minas Gerais, Brazil, an area outside the endemic malaria zone of Brazil. Eight months earlier, there had been transmission of P. vivax for a period of 50 days, which was then totally controlled by chemotherapy and insecticides. An anti-sporozoite response, measured by ELISA using a recombinant protein expressed in yeast, was detected in 45% (14 of 31) of individuals eight months after infection and persisted for 20 months in 12%. Eighteen individuals were treated prophylactically for malaria because they lived in houses in which an overt infection had occurred. Seven of these individuals were ELISA positive; of these, 5 had antibodies against the blood stage parasites. Among 13 other individuals in the endemic area who did not have positive smears, had not been ill, and had not received prophylaxis, five were anti-circumsporozoite positive up to a 40-fold serum dilution. They did not develop asexual blood stage antibodies and remained parasite-free for the following 20 months.