Basement membrane obtained from glomeruli of normal and nephrotic (anti-kidney serum) rats has been examined by microscope electrophoresis and X-ray diffraction techniques. Normal membrane had a mobility at pH 7.35 of 0.93 ± 0.12 µ/s/V/cm; the X-ray diffraction pattern resembled that of collagen, with an additional ring corresponding to a spacing at 2.25 Å. The membrane from nephrotic rats had a mobility of 0.69 ± 0.027 µ/s/V/cm, significantly lower than normal; the mobility was also decreased at pH of 2.25 and 8.75. The X-ray diffraction pattern showed several reflections corresponding to short spacings, not seen in the normal pattern, some of which disappeared on treatment of the membrane with collagenase. It is suggested that the changes in mobility and X-ray diffraction may be the result of loss of phospholipid and that the changes in diffraction pattern result from an increased ordering within the crystal lattice of both collagen and non-collagenous compon