Availability of synthetic and recombinant peptides reproducing the repetitive regions of the circumsporozoite (CS) proteins of Plasmodium falciparum and P. vivax has allowed the development of assays for the detection of specific antibodies and of potential subunit vaccines. Knowledge of the immune responses to malaria sporozoites is a prerequisite for the optimal design of a sporozoite antigen-based vaccine. Studies carried out in areas with stable P. falciparum malaria (United Republic of Tanzania) have shown that antibodies against the synthetic peptide (NANP)40 increase as a function of age. Cluster analysis revealed marked inter-household variation of the anti-sporozoite antibody response, despite comparable risks of exposure to infectious bites. An age-related prevalence of anti-P. vivax sporozoite antibodies has been observed in an area of Sri Lanka with unstable malaria, using a 45-mer synthetic peptide reproducing a defined sequential array of the two main 9-mer variants of the P. vivax CS protein. In this area, anti-(NANP)40 antibodies became detectable after the first epidemic of P. falciparum malaria. Interestingly, their prevalence also increased with age. Since this population had not been exposed to P. falciparum malaria for at least 10 years previously, one can suggest that anti-sporozoite antibodies reflect the relative exposure to infectious bites in the different age groups, and, in turn, the transmission of the disease. This can be particularly useful in areas where entomological indices of transmission tend to be unreliable because of the low vectorial capacity and wide fluctuations in vector densities.