In the present work the Authors report the results of the research of HAAb in 383 subjects living in Rome with negative history for hepatitis (divided for age, sex and occupation) and in 64 acute non B hepatitis (HBsAg-). In the latter, the titer of HAAb was determined at the beginning and later during the course of the disease. A fractionation of 13 sera of these patients was done and the type of specific immunoglobulin was determined. Among the subject with negative history for hepatitis, 68.9% was positive during first 6 months of age 6.2% from 6 months to 5 y.o., 27.6% from 6 to 12 y.o., 45.4% from 12 to 17 y.o., 76.8% from 18 to 25 y.o., 82.2% from 26 to 45 y.o., 90.2% from 46 to 65 y.o.. There was no statistically significant difference either between males and females, or among various occupations. Among the 64 patients with non B hepatitis, 25 (39%) were hepatitis A (because they showed either a seroconversion for HAAb or a positive HAAb-IgM); 12 (18.7%) were non A-non B hepatitis because HAAb negative or HAAb positive but negative for HAAb-IgM; while 27 (42.2%) were impossible to classify because they showed a positivity for HAAb but not a seroconversion during the disease. The limit of the determination of HAAb and the utility of the research of IgM antibody for the diagnosis of hepatitis A are discussed.