In this study, the depressive effects of AFB1 on the embryonic development of chicken thymus were determined by histological methods and by histochemical demonstration of alpha-naphthyl acetate esterase (ANAE) enzyme. ANAE positivity frequencies of peripheral blood lymphocytes (PBL) were also determined. Embryonic stages were determined according to the Hamburger-Hamilton (H-H) scale. Thymic development was quite similar on day 7 of incubation in all control and AFB1 groups. In contrast, the development was substantially impaired and retarded on day 10 of incubation in the 20 and 40 ng/egg AFB1 groups. The ANAE positivity was first detected on the 18th day of incubation in a minority (2%) of PBL. The proportions of PBL and their ANAE positivity frequencies were significantly (P<0.05) lower in AFB1 treated groups than in the controls on the day of hatching.