1. In this study, embryotoxicity and effects of in ovo administration of aflatoxin B1 (AFB1) on the embryonic development of the bursa of Fabricius were determined in fertilised chicken eggs by histological methods and histochemical demonstration of acid phosphatase (ACP-ase) enzyme. Peripheral blood lymphocyte percentages were also estimated. 2. Embryonic stages were determined according to the Hamburger-Hamilton (H-H) scale. Mortality rates increased in AFB1-injected groups in a dose-dependent manner. Embryonic deaths were concentrated at H-H 31 in the 5 ng AFB1/egg group while the deaths were highest at H-H 25 in the 10 and 20 ng AFB1/egg groups. In the 40 ng AFB1/egg group, embryonic deaths mostly occurred during the first 70 to 72 h of incubation (H-H 20). 3. Bursal development was quite similar at d 7 of incubation in control and all experimental groups although development had been substantially impaired and retarded at d 10 of incubation in 10, 20 and 40 ng AFB 1/egg groups. A gradual decrease of ACP-ase positive lymphocyte numbers was observed with increasing AFB1 doses. The chicks in AFB1-treated groups hatched with poorly developed bursae compared with those of the controls. However, proportions of ACP-ase positive lymphocytes substantially increased at hatch and at 21 d of incubation all groups had similar numbers. 4. The results revealed that breeder diets should be investigated for aflatoxins and specifically AFB1, in cases of low hatchability and flock immunity, because low concentration of AFB 1 transferred into the fertilised eggs might be the cause of serious problems.