Large unilamellar liposomes (d approximately 160 nm) composed of dioleoylphosphatidylethanolamine (DOPE) (80-90%), a negatively charged phospholipid stabilizer (10-20%), and a small amount (0.1-1%) of a haptenated lipid are unusually stable in divalent cation-free isotonic buffer at pH 7.4. The liposomes can be stored under this condition at 4 degrees C for at least 6 months without any detectable leakage of the entrapped fluorescent dye calcein. However, the liposomes undergo a rapid (1 h) aggregation and lysis reaction in the presence of free bivalent anti-hapten antibody. The liposome destabilization was immunospecific in that it did not occur with the normal IgG or in the presence of excess free hapten. Liposome lysis was always accompanied by liposome aggregation. Aggregation and lysis of the liposomes was completed in 5 min if the incubation temperature was raised to 70-80 degrees C. Replacing DOPE with dioleoylphosphatidylcholine in the liposomes did not abolish the liposome aggregation, but no liposome lysis was observed even at 80 degrees C. Since liposome aggregation appeared to be a necessary (but not sufficient) prerequisite for liposome lysis, we have named this new class of liposome "contact-sensitive liposomes." The immunodiagnostic potential of the contact-sensitive liposome was demonstrated with liposomes containing theophylline-DOPE. The aggregation and lysis of the liposomes induced by a monoclonal anti-theophylline antibody could be inhibited by free theophylline at concentrations of therapeutic significance. The observation could be the basis of a homogeneous assay for theophylline.