The presence of antibodies against Plasmodium falciparum sporozoites in humans living in malaria endemic areas was measured using as antigen the synthetic peptide (NANP)3, which represents the immunodominant region of the circumsporozoite (CS) protein. The results indicate that: i) the production of anti-CS antibodies is unrelated to the presence in the circulation of blood-stage parasites; ii) anti-CS antibodies, raised by natural inoculation, could exert a protective role against natural malaria infection; iii) anti-CS antibodies can be used as indicators of the intensity of malaria transmission.