IgA nephropathy has been recognized increasingly as a distinct clinicopathological entity. We studied 53 patients with IgA nephropathy immunologically, and clarified a certain relationship between this condition and streptococcal infection. Twenty-three of 42 patients (55%) had the preceeding upper respiratory tract infections. The mean serum concentration of IgA was 323 mg/dl, and 14 of 36 patients (39%) had greater than 350 mg/dl. Anti-streptolysin O titer was elevated in 4 of 47 patients (9%), anti-streptokinase titer in 5 of 33 patients (15%), and anti-hialuronidase titer in 4 of 35 patients (11%). We estimated the titer of antibody to C-carbohydrate derived from streptococcal cell walls which was supposed to persist longer than the antibodies to streptococcal extracellular products. The extraction of C-carbohydrate antigen, i.e. a group specific antigen of streptococcus, was carried out by a modified method of Slade. The quantitative determination of antibody by O-stearoyl C-carbohydrate-sensitized red cell hemmagglutination showed significant high titers in 43% of patients with IgA nephropathy. The results were as follows: titers in the serum of normal adults ranged below 1:16; in IgA nephropathy, between 1:2 and 1:512; in other types of chronic glomerulonephritis, between 1:2 and 1:32; and in anaphylactoid purpura nephritis, between 1:8 and 1:1024. The data above suggested a close relationship between IgA nephropathy and streptococcal infection.