In this study, we characterized the sex hormone receptors in normal and abnormal rat thymus tissues using biochemical and immunohistochemical techniques. Judging from the experimental results, the progestin and the estrogen receptor-containing cells and the thymulin-producing cells are the same reticuloepithelial (RE) cells. This suggests that the sex steroids mediate immune functions of the thymus through receptors within the RE cells to produce thymulin which induces the thymocyte to differentiate and mature. Whether the sex hormones have any direct effects on either T or B cells is not known at this time. So, further studies are needed to clarify this point. Secondly, using the spontaneously developed thymoma tissues from BUF/Mna rat the present authors have just started to biochemically analyse the existence of sex hormone receptors and to immuno-histochemically identify sex hormone receptor-containing cells and thymulin-producing cells. As a contemporary result, progesterone and estrogen receptors were mainly located in the intact RE cells but not in the neoplastic cells, whereas thymulin-producing cells were in both intact RE cells and neoplastic cells. It seems by now that there is no correlation between steroid hormone receptors and thymulin production in the neoplastic cells, as would be in the intact RE cells. Further study is required to answer this question.