Background: Florfenicol (Flo) is one of the widely used antibacterial in food-producing animals. There is controversy about the histological changes in splenic immunocyte induced by Flo. Objective: The proliferation of splenic germinal centers (SGCs) in poultry is considered a marker of an immune response.Therefore, chick embryos were used as a model in our study, as they have a high cellular proliferation rate. Methods: Thirty fertilized eggs were divided into three groups (10 eggs/group), C as control, F1 and F2 treated on day 12 of incubation with distal water, 75 and 150 mg/kg of Florfenicol in  air cell, respectively. Then, on day 18 of incubation, chick embryos were sacrificed for spleen harvesting to examine the histological changes using TUNEL assay (Terminal deoxynucleotidyl transferase dUTP nick end labeling) to detect DNA fragmentation (apoptosis) and hematoxylin and eosin stain to count blood vessels (BVs) and splenic germinal center (SGCs) and their diameter. Chick embryos exposed to Flo led to splenic histological changes in a dose-dependent manner, represented by a significant increase in the number of both BVs and SGCs but the diameter of these SGCs decreased significantly due to the occurrence of apoptosis in the F2 group compared with control and F1 groups. Conclusion: We concluded that Flo induced proliferation in the SGCs of chick embryos, which is considered as an indicator of the immune response that depended in its intensity on the dose and to regulate and modify this response by apoptosis, which is recorded for the first time.