Early alterations relating to lenticular opacity occurred long before large protein aggregates formation in the lens and in plasma membranes of lens fibres. At the first step of human cataract development membranes contained typical knobs, sockets and fragments. Then, with development of cataract, further fragmentation and undulation of lens membranes were observed; surface of the fibre membranes was increased. At the later steps of lenticular opacity partially disintegrated fragments of lens fibres were detected, free membrane structures disappeared and the globules containing protein-lipid complexes of various size and density were formed; these globules were involved in lenticular opacity. With growing of cataract lipid fraction of the lens accumulated primary (hydroperoxides), secondary (diene conjugates and diene ketones) and the end products (Schiff bases) of lipid peroxidation. Ultrastructure modification of membranes in cataract appear to occur in response to accumulation of the lipid peroxidation products. This suggests that lipid peroxidation not only accompany the cataract development but may be among the main pathogenic factors of the disease.