The ability of a purified sea anemone (Stichodactyla helianthus) protein cytolysin to interact with a variety of interfaces was investigated by means of the Wilhemy plate method. At the air:water and lipid:water interfaces, the toxin lowered the surface pressure most readily as the aqueous phase pH increased towards the isoelectric point (9.8) of the toxin. The affinity of the toxin for both the phospholipid:water and the oil:water interfaces was much greater than for the air:water interface. Although the toxin had previously been found to avidly bind to sphingomyelin-containing phospholipid dispersions and bilayers, it failed to display any preferential interaction with a sphingomyelin monolayer relative to one of dipalmitoylphosphatidyl-choline under identical conditions, even when the monolayers were maintained at 40 dynes/cm, a pressure considered to produce phospholipid packing densities similar to those observed in cell membranes. Unlike many other membrane-active protein cytolysins, the ability of Stichodactyla cytolysin to penetrate these phospholipid monolayers was not affected by the initial surface pressure over the range 0-32 dynes/cm. However, at 40 dynes/cm initial packing pressure, the surface pressure generated by the cytolysin was similarly reduced in both sphingomyelin and dipalmitoylphosphatidylcholine monolayers. Our results suggest that the protein cytolysin initially binds reversibly to cell and artificial bilayer membranes in a non-specific manner; sphingomyelin domains in the bilayer then provide optimal conditions for insertion into the membrane and subsequent assembly of a stable multimeric complex which functions as an ion channel.