To study mechanisms whereby lytic proteins disrupt biomembranes, artificial phospholipid spherules (liposomes) were exposed to melittin, a cationic peptide in which sequences of hydrophobic (Positions 1 to 20) and hydrophilic (Positions 21 to 26) amino acids are unequally distributed. At concentrations above 10⁻⁶M, melittin, which is the major toxin of bee venom, released marker anions (CrO₄⁼) or glucose from the model structures. Release of marker ions was equivalent whether liposomes were prepared with a net negative (dicetyl phosphate) or net positive (stearylamine) charge. Nor was the presence of cholesterol in the model membranes necessary for the action of melittin. Spherules with net negative charges competed approximately 10 times more effectively for melittin's lytic activity on erythrocytes than did spherules with positive charges. Studies with lipid monolayers indicated that melittin had a remarkable affinity for the air-water and more so for the lipid-water interface, for it spread as a film readily from water and penetrated lipid monolayers avidly, irrespective of the surface charge of the lipid film. Negatively stained preparations of liposomes in the electron microscope showed that melittin-treated spherules lost the integrity of their concentric lamellae, which became beaded, frayed, and finally fragmented. These studies indicate that the lytic effects of melittin upon biological membranes may be due to a similar reaction with structural phospholipids. Since ionic interactions between the cationic peptide and charged lamellae were not crucial for lysis, and since melittin has an extraordinary affinity for lipid membranes, it is suggested that the surface activity of melittin and convenient apolar associations between hydrophobic portions of melittin and the acyl chains of phospholipid could account for disruption of the spherules.