Twenty-one cases of acute glomerulonephritis in children with no previous history of renal disease were studied. Urinary infection with a rising titre of serum agglutinins against the organisms isolated from urine was found in 5 cases. No evidence of previous streptococcal infection was found in these cases. In the meantime all 8 cases with post-streptococcal glomerulonephritis remained without bacteriuria. In one case acute glomerulonephritis followed virus hepatitis, and in the remaining 7 cases the cause of glomerulonephritis was unknown. It is suggested that in predisposed patients the bacteria present in urinary infections might act as antigens starting immunologic reactions in the glomeruli, leading to glomerulonephritis. The final proof of this theory awaits immunofluorescence identification of these antigens in the glomeruli.