To determine a regulatory function of natural antibodies in the immune response of chickens, pooled plasma obtained from non-immunized (naïve) 15 months old hens was subjected to keyhole limpet hemocyanin (KLH) antigen-affinity chromatography. Purified KLH-binding antibodies were adoptively transferred intravenously to 5 weeks-old cocks that were subsequently immunized subcutaneously 24 h later with KLH. Control groups consisted of birds that were either adoptively transferred with KLH-binding antibodies purified from plasma of KLH-immunized chickens, or PBS, or a salt precipitated total immunoglobulin fraction obtained from the corresponding pooled nai;ve chicken plasma, respectively.Total, IgM and IgY antibody titers to KLH in the plasma of recipients adoptively transferred with KLH-NAb, but not in the plasma of the groups transferred with salt precipitate or KLH-binding specific antibodies, were significantly enhanced as compared to the non-treated, KLH immunized group. Titers of IgA antibodies binding KLH were decreased in the plasma of the group that received specific KLH-binding antibodies, but not in the plasma of the other groups. Proliferation from peripheral blood leucocytes in whole blood from the KLH-NAb treated group, the group treated with KLH-binding specific antibodies and the group treated with salt precipitate, respectively, to both concanavalin A and KLH were significantly decreased as compared to the group receiving PBS. Our data show that antigen-specific antibodies can be isolated from plasma obtained from non-immunized chickens. Such antibodies that resemble natural antibodies as described in mammals may perform an important role in the enhancement of subsequent antigen-specific antibody responses or the maturation of the immune system, which may differ from the role of specific antibodies.