Autoantibodies against a variety of growth factors and cytokines are present in preparations of pooled normal human IgG, such as IVIg. The present study demonstrated that healthy Danish blood donors produced high concentrations of anti-IL-10 IgG antibodies that bound IL-10 with extremely high avidity. The antibodies were of IgG class, polyclonally derived and acted as competitive IL-10 inhibitors in vitro, substantially inhibiting cellular IL-10 receptor binding and neutralizing IL-10 activity in vitro. The antibodies failed to bind viral forms of IL-10 or other members of the human IL-10 family (IL-19, IL-20, IL-22, IL-24, IL-26, IL-28A, IL-28B, IL-29). The production of anti-IL-10 antibodies was stable from months to years, and high positive donors were likely to acquire a state of IL-10 deficiency in the circulation during this period. Anti-IL-10 antibodies were readily measurable even in highly diluted plasma samples, providing the explanation for the fact that relatively low antibody activity can be detected in normal human pooled IgG, derived from the plasma of over 1000 blood donors.