The optimal time of initial vaccination with a mild-type live IBD vaccine in chicks with maternally derived antibody (MDA) was studied. The half-life of MDA was 3.46 days, and critical MDA titer to protect from infection with field virus was 1: 28. The protective immunity was acquired 6 days after vaccination, and an MDA titer of 1: 28 interfered with active immunization. If the initial vaccination should be made at age when geometric mean MDA titer lowered to 1: 80, the optimal time (Y) was calculated from age of MDA titration in days (Z) and log₁₀ geometric mean MDA titer (X) of flocks by the following formula; Y=11.49X+Z-21.87.