Morphologic studies have revealed that particles of endotoxin (LPS) associate with membranes by edge attachment. Also, under certain conditions endotoxin disks or lamellae may form vesicles on interaction with phospholipid. Edge attachment of LPS to membranes probably allows the lipids of LPS to interact with membrane receptors or lipids, and the vesiculation reaction provides a possible model for the incorporation of LPS into a membrane.