AbstractThe patients and staff members of a haemodialysis unit were examined for their serological responses to SO‐antigen, which was isolated from the urine of epidemic type non‐A, non‐B hepatitis patients at Tohoku University Hospital. To understand how SO‐antigen or SO‐antigen‐related aetiology can be incriminated for the hepatitis found in the haemodialysis unit, the prevalence of SO‐antigen/anti‐SO system and hepatitis A and B virus‐related antibodies was compared in the sera of patients and staff members. Although the SO‐antigen was rarely detected in the serum, anti‐SO antibody was frequently detected in the sera of patients and staff. A significantly higher prevalence was found in the serum of patients (15%, 54 out of 361) than staff members (7.1%, 13 out of 184) and volunteer blood donors (1%, 3 out of 305). The same prevalence percentages of HBV‐related antibodies (either positive for anti‐HBs or anti‐HBc) and anti‐HAV were observed among the patients, staff, and volunteer blood donors, irrespective of whether the sera were anti‐SO positive or negative.Among the staff, anti‐SO antibody was more frequently found in those with a history of acute hepatitis (16.7%, 3 out of 18) than in those without (6%, 10 out of 166). These prevalence ratios conformed with those of HBV‐related antibodies, but the same prevalence ratios of antibody to HAV were observed between the staff with and without a history of acute hepatitis.These results indicate that the SO‐antigen/anti‐SO system or entity related to this immune system is distinct from HBV or HAV, and this immune system was found widely in the haemodialysis unit where type B and non‐A, n