Egyptian J. Anim. Prod. (2011) 48(1):77-85. Issued by The Egyptian Society of Animal Production ANTIBODY RESPONSE IN EARLY HATCHED CHICKS AS INFLUENCED BY VACCINATION AND DIFFERENT MATERNAL ANTIBODY LEVELS A. S. Ahmed Department of Animal and Fish Production, College of Agriculture and Food Science, King Faisal University, Saudi Arabia SUMMARY This study evaluated the antibody response to Newcastle disease (ND) and sheep red blood cells (SRBC) of early hatched chicks as influenced by their maternal immune capacity and early vaccination. Chicks of the current study were produced using 32 week old three parent groups of local Saudi chickens which had been sorted in a previous experiment into high (H) , low (L) and a random control group (C) according to their primary antibody response at 7d post im injection with 1ml of 15% SRBC. Parents were vaccinated late at 31 weeks of age against ND and secondarily exposed to SRBC antigen at 32 wk of age. Three hatches from each group were obtained. Half of each hatch was vaccinated at 3 days of age with ND vaccine (HB1) and SRBC. The other half was left without any vaccination. Antibody titer for all chicks against ND and SRBC were evaluated at 6, 9, and 12 days of age. Body weights were recorded weekly and weight gain then calculated. The L group chick body weight, and weight gain, were significantly (P< 0.05) higher than H group at all measuring point. The unvaccinated chicks of H group were significantly (P< 0.05) higher than L group for ND titer at all measuring point and for SRBC at the first two measuring points. The current study demonstrated that the chick antibody response was influenced by the maternal antibody level for SRBC and ND antigen. Results of body weight and weight gain emphasized the concept of allocation of resources between the different needs of one bird. The study spotlighted the importance of considering maternal antibody level before designing a vaccination program for early hatched chicks. Keywords: Newcastle disease, sheep red blood cells, maternal immunity, body weight INTRODUCTION There are many reports in literature over time regarding chick susceptibility to many pathogens during the first few weeks of age because their immune system is not fully developed; hence, maternal antibodies are the primary means of antigen- specific protection until adaptive immune responses become fully effective (Hamal et al., 2006; Davison et al., 2008).The immunoglobulin Y (IgY) is selectively secreted via the circulation of the hen into egg yolk (Patterson et al., 1962). The amount of IgY transferred across the follicular epithelium into the yolk is proportional to the IgY concentration in serum (Al-Natour et al., 2004; Hammal et al., 2006). The IgY is continuously absorbed by embryo during embryogenesis until