In senile cataractous nucleus extensive areas occur containing degenerated and dissociated fiber membranes and aggregated protein. Globules of 100–600 nm diameter are found containing aggregated protein and frequently without a surrounding membrane. During ageing, phosphoglycerides are broken down, which induces a high cholesterol-phospholipid ratio ( > 2.6) and high microviscosity in the fiber membranes. The resulting increased exposure of membrane proteins to the cytoplasm may enable oxidation of previously hidden thiol groups and disulphide bridge formation between cytoplasmic and membrane proteins. Extensive protein-membrane aggregates have been isolated from the cataractous lens nucleus. They are urea-insoluble and can be dissociated by reducing agent. 83% of their phospholipid consists of sphingomyelin. A part of the membrane lipids has been transferred to the soluble cytoplasmic protein fraction, while membrane lipo-proteins have lost their original tight bond to the other components.