The persistence of antibody to hepatitis A antigen (anti-HAV) of the immunoglobulin M (IgM) class was evaluated in 88 sera of 51 acute hepatitis A patients. IgM was separated from IgG by a 2-h reorienting sucrose gradient ultracentrifugation, and the titer of anti-HAV was determined in the IgG- and IgM-containing fractions by solid-hase radioimmunoassay. IgM anti-HAV was the predominating antibody at onset of jaundice and persisted in these patients for at least 60 days, but not longer than 115 days. The demonstration of IgM anti-HAV is therefore a valuable tool for the diagnosis of recent hepatitis A infection.