Hepatitis A antibody (anti-HAV) was detected by specific radioimmunoassay (RIA) method in sera from 10 patients with acute icteric hepatitis. Anti-HAV was detectable in many subjects very early before the onset of jaundice, but the diagnosis of type A hepatitis in all patients was confirmed by the demonstration of seroconversion during convalescence. Since the initial antibody detected by RIA is predominantly IgM, while IgG specific anti-HAV appears later reaching peak levels within 1 to 2 months, we treated serum specimens of these patients with 2-mercaptoethanol (2ME) in order to differentiate acute-from convalescent-phase hepatitis A sera. Inactivation of IgM fraction with 2ME produced a significant reduction of anti-HAV titer only in acute-phase sera, so that this procedure may be used for early diagnosis of acute type A hepatitis.