To the Editor:— This is a report of studies of anti-A and anti-B titers in children in the age range of 3 to 36 months. Older studies were made by methods little used today.1-3Anti-A and anti-B titers may be helpful for comparative purposes when the diagnosis of hypogammaglobulinemia or agammaglobulinemia is under consideration. Materials and Methods:— Blood specimens sufficient in size for titrations were obtained whenever possible from children in cases of disputed paternity. All children appeared healthy at time of venipuncture. As adult controls, parents (or alleged parents) were tested in the same instances. Each serum sample was tested against fresh A1, fresh B, and fresh O cells, by mixing 0.1 cc of serum (or serum dilution) under test with 0.1 cc of 2% cell suspension in 0.9% sodium chloride, shaking, letting stand one minute at room temperature, centrifuging at 1,000 rpm for one minute, resuspending by gentle