Lymphocytes play essential roles as a kind of leukocyte in the defense mechanism of animals against infectious factors. Lymphocytes are classified into two subsets, T lymphocytes and B lymphocytes in mammals or avian species. In avian species, T lymphocytes differentiate and proliferate in the thymus which is a central lymphoid organ common to mammalians and avian species, whereas on the other hand, B lymphocytes have been known to occur in the bursa of Fabricius (bursa) which is a unique central lymphoid organ of birds. Steroid hormones, such as androgen and estrogen, have been reported to change differentiations and proliferations of these lymphocytes in corresponding lymphoid organs, indicating steroid hormones give influence lymphocyte development positively or negatively in the bursa and thymus of birds. Studying the relation between steroid hormones and lymphocyte development in the central lymphoid organs is important, because changes in the lymphocyte genesis in central organs of birds may result in altered levels of antibody production and immune functions related to T lymphocyte activity. We have studied effects of androgen and estrogen on lymphocyte differentiation and proliferation in the central lymphoid organs of chicken at Okayama University since the 1980s. In the present report, the results of these studies are summarized.