We demonstrate that antimicrobial peptides induce an autophagic cell death in the protozoan pathogen, Leishmania donovani. In our study, three antimicrobial peptides, Indolicidin, and two peptides derived from Seminalplasmin exhibit antileishmanial activity with a 50% lethal dose of 3.5 x 10(-5), 3.8 x 10(-4) and 1.7 x 10(-8) microM, respectively. The action of these antimicrobial peptides on the Leishmania cell involves ionic interactions, which are modulated by lipophosphoglycan on the parasite's surface. Peptide treatment caused dissipation of membrane potential and equilibration of intracellular pH with extracellular environment. However, there was no release of intracellular GFP molecules upon peptide treatment of a GFP expressing Leishmania clone. Transmission electron microscopic studies show extensive intracellular damage including cytoplasmic vacuolization and degeneration of cellular organization without disruption of the plasma membrane. These peptides induce cell death via a non-apoptotic process as shown by lack of nuclear fragmentation or DNA laddering and independent of caspase-like activity. Instead, Monodansylcadaverine (MDC), a biochemical marker of autophagy specifically labels the vacuoles induced by peptides. Collectively, these results indicate that in addition to their effects on the leishmanial membrane, these antimicrobial peptides induce pathway(s) for autophagic cell death in L. donovani.