For the purpose of studying the relationship between streptococcal infection and renal diseases, we estimated the titer of antibody to streptococcal C-carbohydrate in serum of patients with various renal diseases. The results were as follows: serum titers were less than 1:8 in normal adults, less than 1:16 in normal children, between 1:16 and 1:1024 in acute glomerulonephritis, between 1: 8 and 1: 1024 in anaphylactoid purpura nephritis, between 1:2 and 1:512 in chronic glomerulonephritis, between 1:4 and 1:8 in primary nephrotic syndrome, and between 1:2 and 1:16 in lupus nephritis. This results showed significant high titers in acute glomerulonephritis, anaphylactoid purpura nephritis and chronic glomerulonephritis, suggesting a close relationship between these diseases and streptococcal infection. The titers in patients with IgA-nephropathy which was included in chronic glomerulonephritis in the present study were higher than in patients with other groups of chronic glomerulonephritis. This may suggest that streptococcal infection plays an etiologic role in IgA-nephropathy.