The human thymic microenvironment is important in promotion of T cell maturation, particularly during early stages of thymic ontogeny. Hassall's bodies (HB) are epithelial swirls in the human thymic medulla that are thought to be derived from endocrine medullary thymic epithelium. To study the ontogeny and function of various components of the human thymic microenvironment, we have produced four monoclonal antibodies (TE-8, TE-15, TE-16, and TE-19) that selectively reacted in thymus with HB. Antibodies TE-8 and TE-16 reacted with the cells forming the outer rim of the HB swirl. Antibody TE-19 reacted with the entire cellular portion of HB and with epithelial cells immediately surrounding HB. Granular foci in the cellular swirls of greater than 90% of HB reacted with antibody TE-15. During thymic ontogeny, the antigens defined by antibodies TE-8, TE-15, TE-16, and TE-19 were first detected in fetal thymus on HB beginning at 16 wk gestation, the age when HB morphologically appear in the thymus. Aberrant expression of the antigens corresponding to antibodies TE-8, TE-15, TE-16, and TE-19 was observed on thymic tissue from individuals with severe cellular immunodeficiency disease. In human skin, antibodies TE-8, TE-16, and TE-19 reacted with the stratum granulosum; antibody TE-15 reacted with the stratum corneum. Thus, with the use of antibodies TE-8, TE-15, TE-16, and TE-19, we have identified HB as antigenically distinct regions of endocrine thymic epithelium. Furthermore, we have shown that these anti-HB reagents also selectively react with epidermal keratinocytes in the terminal stages of keratinocyte maturation.