Background: Sunset yellow (E110) has largely been used as food coloring agent. Complaints accumulated on E110 for possible detrimental effects on the immune functions such as allergenicity in the children. In this study, the effects of different doses of E110 on embryonic development of chicken primary lymphoid organs, thymus and bursa of Fabricius were determined by means of histological, histomorphometrical and histochemical methods. A total of 250 fertilized eggs from the Ross 508 line were used in the study. The eggs were divided into 5 groups as non-treated, sham-exposed and 100 ng/egg, 500 ng/egg, 1000 ng/egg E110-injected groups, each having 50 eggs. Results: In the 1.000 ng/egg E110-administered group, embryonic development of chicken thymus and bursa of Fabricius were retarded. Conclusions: E110 given in ovo before incubation retarded the embryonic development of the avian thymus and bursa of Fabricii. The effect is more pronounced in the 1.000 ng/egg group.