The purpose of this study was to investigate the interaction of polyvinyl pyrrolidone (PVP) with phospholipid bilayers in an effort to add a new dimension to our understanding of polymer interaction with lipids. For the preparation of lipid-polymer dispersions, measured amounts of DPPC (1,2 dipalmitoyl-sn-glycero-3-phosphocholine) and PVP were mixed in chloroform. After complete elimination of organic solvent, the dry mixtures were hydrated at 50-55 degrees C. Interactions between DPPC and PVP were assessed by differential scanning calorimetry (DSC) and photon correlation spectroscopy (PS). Separation of liposomes and micelles was performed by centrifugation. Liquid scintillation counting and a UV spectrophotometer were used for their analysis. PVP added as dry powder or added as aqueous solution to dry lipid or preformed liposomes failed to interact. Only PVP previously dissolved in chloroform interacted with DPPC. The DPPC main phase transition moved to lower temperatures with increasing PVP concentrations. This reduction of the phase transition temperature was accompanied by an increase of the DPPC phase transition enthalpy. Analysis of solubilization indicated that the amount of PVP present in the bilayer is dependent on the PVP bulk concentration. The data suggest interaction of PVP previously dissolved in chloroform with the acyl chains of the phospholipid deep into the bilayer.