Introduction.Since the first reports of visceral leishmaniasis in Brazil, the sand fly Lutzomyia longipalpis has been incriminated as the main vector of Leishmania infantum, the causative agent of visceral leishmaniasis in the Americas.The vector saliva modulates the host immune response at different levels, including inhibition of the complement system.This salivary activity may be critically important for Leishmania transmission from the vector to the host.This study aimed to demonstrate the importance of salivary inhibitors of the complement to the survival of promastigote forms of L. infantum when exposed to the complement system of human or dogs infected or non-infected by Leishmania.Material and methods.In the presence of propidium iodide, L. infantum parasites were incubated with different concentrations of human or dog serum, containing or not different concentrations of L. longipalpis' saliva.Along six minutes, aliquots of each group were withdrawn at stated times, to perform the kinetics of cell death by flow cytometry (dead cells in the presence of propidium iodide emit fluorescence).Results.In the presence of human serum, we observed significant reduction of cell death when saliva of the vector is added.This reduction was dependent on the concentration of saliva and serum of the host.However, both the serum of infected and the uninfected dogs were not efficient in the lysis of the parasites at six minutes of incubation, even in the absence of salivary extract. Conclusion.The vector saliva enhances the survival of promastigotes of L. infantum when exposed to human serum.It may be crucial in the process of transmitting the parasite to the human host.The high resistance of the parasite to the lytic effect of dog serum could explain why the dogs are more susceptible to visceral leishmaniasis. Infección natural de Lutzomyia evansi con especies del complejo Leishmania braziliensis causantes de leishmaniasis cutánea en la Costa Caribe colombiana