Sand fly saliva contains multiple factors with distinct pharmacological activities. We have previously cloned and functionally expressed a cDNA that encodes one such factor, a highly potent peptide vasodilator called erythema-inducing factor (EIF), which creates the characteristic red spot at the site of a bite. Characterization of EIF cDNAs from numerous different geographic isolates of sand fly indicate a high degree of polymorphism in the coding DNA with as much as 13 of 63 amino acid differences. Yet the vasodilatory specific activity of the recombinant proteins shows no significant differences. The sand fly also transmits human leishmaniasis which is caused by the protozoan parasite, Leishmania, and salivary factor(s) are known to exacerbate infectivity, often by 10-100-fold. The parasites infect macrophages at the site of the fly bite, from which they replicate and spread. The clinical manifestations of leishmaniasis range from self-healing cutaneous lesions to a potentially fatal visceral form depending upon the species of the parasite and the immunological response of the host. Recently, we demonstrated that recombinant EIF, in addition to inducing erythema, can exacerbate experimental leishmaniasis. We then tested whether protein or naked DNA vaccination against this peptide will neutralize the exacerbative effects of saliva. Either immunization method induced both a humoral and cellular response to EIF. Immunized animals were then challenged by foot pad infections of 10 5 Leishmania major parasites in the presence of sand fly saliva. Two and four weeks post-infection, all vaccinated mice were found to have markedly reduced levels of parasites within their foot pads compared with control animals. Immunization completely reversed the enhancing effect of saliva on Leishmania infectivity, even when the saliva contained EIF with many polymorphisms relative to the immunogen. These preliminary results suggest that EIF may be a target for vaccination against naturally transmitted Leishmania.