Basement membrane prepared by ultrasonic vibration of isolated glomeruli was obtained from normal rats and from rats with anti-kidney serum nephrosis. The membrane was found to contain collagen, lipids, and at least two glycopeptides which were separable by chromatography on Seρhadex-G75 after removal of collagen by collagenase. Sialic acid was associated with collagen and with one of the glycopeptides. Acid hydrolysis released all the sialic acid in a dialyzable form. Neuraminidase released the same amount of sialic acid but only half was dialyzable, indicating that the remainder was not present in a terminal position. Nephrotic membrane contained less phospholipid and acetyl than normal, but the same amount of collagen, hexosamine and sialic acid. The sialic acid was liberated more readily by chemical hydrolysis, but less readily by neuraminidase than that of normal membrane; the collagen was also more readily degraded. It is concluded that in nephrosis, the amount of basement membrane material is not increased and that the membrane thickening seen microscopically, and the increased permeability, result from a change in organization at a molecular level