AbstractTwo hundred one immune serum globulin (ISG) lots manufactured in the US between 1967 and 1977 were tested for antibodies to the hepatitis A virus (anti‐HAV) by a competitive‐inhibition radioimmunoassay (RIA); a lesser number were also tested by immune adherence hemagglutination (IAHA). The percentage of ISG lots that contained anti‐HAV with a titer of 1:100 or greater by RIA was 50% for those manufactured in 1967, 69% for those manufactured in 1972, and 100% for those manufactured in 1977. The percentage of lots with anti‐HAV titers equal to or greater than 1:500 by RIA was 7% in 1967, 18% in 1972, and 70% in 1977. Only ten lots of ISG (5%) had anti‐HAV titers of 1:1,000 or greater by RIA; seven of these were manufactured in 1977.Both the mean titer of anti‐HAV in ISG lots and the percentage of lots containing significant titers of this antibody appear to have increased in the US over the past ten years. This may reflect the increased use of source plasma from paid plasmapheresis donors in the US during this period. The lower titers of anti‐HAV in the older lots of ISG studied were shown not to be due to fragmentation of antibody molecules d