The localization of radiolabeled estradiol and dihydrotestosterone was examined in the lymphatic organs of both male and female baboons. A total of 12 baboons were divided into two groups, each containing three males and three females. Each animal in one group, both males and females, was injected intravenously with 1 microgram/kg body weight of 3H-estradiol while those in the second group were each injected with 1 microgram/kg body weight of 3H-dihydrotestosterone. As controls, one male and one female from each group also received a dose of 100 micrograms/kg body weight of the corresponding unlabeled steroid. One and a half hours after the injections, the animals were sacrificed and the spleen, thymus, and inguinal lymph nodes removed and processed for autoradiography. The localization of 3H-estradiol was similar in both males and females. In the thymus fibroblasts and epithelio-reticular cells, but not thymocytes, localized 3H-estradiol. In lymph node and spleen, non-lymphoid tissue concentrated the labeled estrogen. Additionally, in the paracortical region of the lymph node, an unknown cell type was labeled with estrogen. Only one male baboon demonstrated nuclear localization of 3H-dihydrotestosterone. This was observed in the reticular cells in the spleen and lymph nodes. The same cell type in the organs of the remaining animals was unlabeled.