In this study, the effects of aflatoxin Bı (AFBı), in ovo administrated,on the embryonic development of chicken spleen was evaluated. For this purpose, 651 laying hen's eggs were divided into 7 groups. These groups were designed as control-1, control-2, 30% ethyl alcohol (ETOH), 5 ng/egg AFBı, 10 ng/egg AFBı, 20 ng/egg AFBı and 40 ng/egg AFBı. Test solutions were injected via the air sac at pre-incubation period. Six eggs from each groups were opened at 12nd, 13rd, 15tfl and 18th days of incubation. At these periods and at the day of hatching, splenic tissue samples from were collected and processed for routine histological techniques. Light microscopic investigations of the tissue preparations have revealed that embryonic development of the chicken spleen was substantially impaired and, white pulp regressed when compared those of the controls. At the day of hatching, development of spleen taken from 5 and 10 ng/egg AFB₁ groups was incomplete. It was concluded that small amounts of AFB₁ found in fertilised avian egg has detrimental effects on the embryonic development of chicken spleen.