Polyclonal and monoclonal antibodies to liposomes having various phospholipid compositions have been produced. Binding of the anti-lipid bilayer antibodies is influenced both by the chemical composition and the physical state of the liposomal lipids. The antibodies to liposomes have a 'subsite' in the binding site that recognizes small soluble phosphorylated haptens such as nucleotides (e.g., ATP). The capacity of anti-liposome antibodies to bind to phosphate is also shared by antibodies to numerous other substances, including lipid A from Gram-negative bacterial lipopolysaccharide, cardiolipin, DNA, polynucleotides, and lipoteichoic acids from Gram-positive bacteria. Because of similarities of chemical structures between all of these molecules widespread immunological cross-reactivities are observed.