Thermodynamic characterization using high sensitivity differential scanning calorimetry has been carried out in the following areas: Phospholipid membranes: Cyclosporine A partitions between gel and liquid-crystalline phases of dipalmitoylphosphatidylcholine bilayers in a non-ideal manner. This behavior arises primarily from excluded volume effects. The effects of unsaturated anesthetic alcohols on the bilayer phase transition behavior points out a significant role of van der Waal's forces in the pretransition to the ripple phase. Both studies indicate that the behavior observed in the bilayer model does not always mimic the behavior of these compounds in cellular systems. Proteins: At subsaturating concentrations of fatty acid ligand, the thermal denaturation profile of human serum albumin may, in the case of strong binding (palmitate), be bimodal or in the case of weaker binding (caprylate) be unimodal. A theory to account for these general effects in scanning calorimetry has been developed. Macromolecular Assembly Processes: The complex thermal denaturation profile of the bacteriophage T4 head particle in the large particle expanded lattice form has been interpreted.