Natural, often autoreactive antibodies are present in normal sera in large quantity and show alterations in specificity in diverse pathological situations. They have, however, usually not been studied longitudinally. Here we investigated some representative serum reactivities of natural antibodies in 67 normal children and 10 with injury during childhood, followed up for 3 years. Normal children showed an individually characteristic and relatively stable level of most IgM, IgG, and IgA reactivities when measured with ELISA by reference to a standard. Injured children showed some very rapidly enhanced reactivities within 3 days after trauma, which thereafter slowly diminished over years before coming back to a normal level. This period exceeds by far the lifetime of antibodies and plasma cells. We conclude that natural antibodies contribute to the establishment and maintenance of immune memory in a manner that is distinct from classical immune reactions.