Membrane preparations from normal and cataractous human lenses were prepared by a procedure which minimally disrupts membrane lipid structure. Fluorescence depolarization ofcisandtrans-parinaric acid probes was measured as a function of temperature in membranes, lipid extracts, and phospholipids. These measurements indicated that membranes from cataractous lenses were less ordered (more fluid) than equivalent membranes from normal, age-matched control lenses. No significant differences could be detected between normal and cataractous membrane total lipid extracts, or between normal and cataractous membrane phospholipids. These observations imply that membrane lipid-protein interactions play a significant role in determination of lens membrane structure, and that these interactions are altered in cataractous lens membranes.