An enzyme-linked immunosorbent assay (ELISA) based on the synthetic peptide (NANP)40 was used to characterize the sporozoite antibodies in an unusual Plasmodium falciparum outbreak in a non-malarious area in Sri Lanka. A positive antibody response was seen in 62% of patients with their first P. falciparum illness. There was no correlation between sporozoite antibodies and the antibody against blood stages, determined by immunofluorescence assay. The majority (91%) of the patients lost the antibodies to circumsporozoite (CS) protein within one year (in the absence of re-exposure). Three patients had high levels of CS antibodies even after one year, and this persistence was related to the level of the initial antibody response. In the area of the outbreak 10% of schoolchildren had antibodies to the (NANP)40 peptide. 21% of the 42 children with present or past overt malaria were antibody positive. Of the children with no such background, 8% were antibody positive. The corresponding seropositivity rates for asexual blood stages were 31% and 1% for the 2 groups respectively. It is concluded that (NANP)40 ELISA is potentially a valuable tool in sero-epidemiology, particularly in situations of seasonal transmission and recurrences due to drug resistance.