Although the inhibitory effect of maternal antibodies on active immunization of neonates has been extensively documented, much less attention has been devoted on the exact level of these antibodies which can induce this effect and the extent of such effect. Firstly, laying hens were immunized with dinitrophenyl-keyhole limpet hemocyanin (DNP-KLH).Then, maternal anti-DNP antibodies in chicks derived from these hens were measured by using enzyme-linked immunosorbent assay (ELISA). Chicks with high levels of maternal anti-DNP showed immune suppression, while chicks with low levels of maternal anti-DNP showed normal immune response when they immunized with the same antigen at 1 and 4 weeks of age. Then, different doses of purified maternal anti-DNP were transferred to fertile eggs at 16 days of embryogenesis by in ovo injection and all chicks were immunized with DNP-KLH at 1 and 4 weeks of age. Chicks received 1 mg of anti-DNP showed normal immune response, chicks received 3 mg of anti-DNP showed weak immune response, and chicks received 5 and 8 mg of anti-DNP showed immune suppression. Chicks received 8 mg of anti-DNP were immunized with DNP-KLH at 4 and 7 weeks of age. Their immune response was significantly lower than that of chicks of no-maternal anti-DNP. These results suggested that high levels of maternal antibodies interfere or suppress the immune response of active immunization not only at early period but also at the period in which the maternal antibodies at very low levels.