X-ray small-angle diffraction, differential scanning calorimetry (DSC), and temperature scanning densitometry (TSD) were used to study the effect of b-lysin on the phase transitions of lipid assemblies from 1,2-0-dixehadecyl-sn-glycero-3-phosphoholine (DHPC). The experiments were carried out in excess of water in a temperature range of 0-55 ~ and at low peptide concentrations between 10 -4 and 10-2 moles peptide per mole phospholipid. The incorporation of b-lysin into lipid assemblies alters the lipid structure without significant changes on the temperatures of phase transition from gel to liquid crystalline phase. The temperature of the main transition was nearly unaffected. A reduction in the transition volume of the lipids with increasing concentrations of b-lysin was observed. The minor changes in these parameters were interpreted as long-range structural changes caused by the peptide incorporation. The results are discussed in terms of the concept of cooperative phase transition of entire clusters occurring within a membrane implying that relative stable domains of gel phase, and liquid crystalline phase co-exist.