The Kidney auto-antibody was measured by the complement fixation test on the 92 cases in kidney diseases, mainly diffuse glomerulonephritis and 305 cases in other various diseases and the clinical significance was studied, And the following results were obtained. 1. The positive rate of the kidney auto-antibody in diffuse glomerulonephritis was 48.4% at the acute stage and 80.0% at the chronic stage, but it was 20.0% in nephrosis, 25.0% in kidney tuberculosis, 20.0% in kidney tumor and 33.3% of the kidney in pregnancy. 2. It was 18.3% in various diseases other than kidney diseases, and 16.6% in acute tonsillitis, 20.0% in chronic tonsilitis, 20.8% in essential hypertension with albuminuria, 8.7% in that without albuminuria, and it was shown a little high rate in liver diseases, such as 18.6% in acute hepatitis, 28.3% in chronic hepatitis and 59.0% in livercirrhosis, but the peculiarity to the each antigen was relatively strong by the crossing test with the liver and kidney antigen. The diseases, other than the above diseases, showing the rather high positive rate of kidney auto-antibody were 22.2% in diabetes mellitus and 66.6% in collagen disease. 3. In diffuse glomerulonephritis, it was low at the acute early stage and it gradually elevated since about one month later and it showed the tendency of decrease since two month later at which time it was the highest, but it was generally high at the chronic stage. 4. The serum complement titer showed relatively high decrease at the acute stage of diffuse glomerulonephritis and it showed a little decrease in most of the chronic cases, and the correlation between the kidney auto-antibody and the complement titer was observed. 5. The correlation between the kidney auto-antibody and the findings of various clinical tests, for example urinary change, glomerular filtrating value, nonprotein nitrogen and the increase of serum γ-globulin fraction was observed. 6. The vicissitude of the kidney auto-antibody became the characteristic of the clinical cause and prognosis in diffuse glomerulonephritis, and it also agreed with the histological change of kidney. 7. The kidney auto-antibody moved into urine in the form not loosing the activity and its antibody contained in the γ-globulin of urinary ablumin.