Trypanosome lytic factor 1 (TLF1) is a subclass of human high-density lipoprotein that kills some African trypanosomes thereby protecting humans from infection. We have shown that TLF1 is a 500 kDa HDL complex composed of lipids and at least seven different proteins. Here we present evidence outlining a new paradigm for the mechanism of lysis; TLF1 forms cation-selective pores in membranes. We show that the replacement of external Na+ (23 Da) with the larger tetramethylammonium+, choline+ and tetraethylammonium+ ions (74 Da, 104 Da and 130 Da) ameliorates the osmotically driven swelling and lysis of trypanosomes by TLF1. Confirmation of cation pore-formation was obtained using small unilamellar vesicles incubated with TLF1; these showed the predicted change in membrane potential expected from an influx of sodium ions. Using planar lipid bilayer model membranes made from trypanosome lipids, which allow the detection of single channels, we found that TLF1 forms discrete ion-conducting channels (17 pS) that are selective for potassium ions over chloride ions. We propose that the initial influx of extracellular Na+ down its concentration gradient promotes the passive entry of Cl- through preexisting Cl- channels. The net influx of both Na+ and Cl- create an osmotic imbalance that leads to passive water diffusion. This loss of osmoregulation results in cytoplasmic vacuolization, cell swelling and ultimately trypanosome lysis.