Human fetal thymic epithelial cells (TEC) were cultured under serum-free conditions. The TEC were analyzed for the secretion of 14C-labelled peptides and of IL-3, IL-6 IL-7, IL-8, GM-CSF, LIF, fibronectin and thymosin alpha 1 by ELISA tests. IL-3 and IL-7 were not detected from these TEC. Lack of IL-7 and presence of thymosin alpha 1 and of the surface molecule TE-4 depicts these cells as subcapsular/medullary TEC. TEC secreted constitutively IL-6, IL-8, fibronectin and thymosin alpha 1 but not GM-CSF of LIF. Stimulants included recombinant IL-1 and monoclonal antibodies to the surface adhesion molecules ICAM-1 and LAF-3. In addition, hydrocortisone (2.7 x 10(-7) M) was used to dissect secretion patterns. Recombinant IL-1, anti ICAM-1, and anti LFA-3 alone and collectively induced modest but significant increases in the secretions of 14C-labelled peptides and of IL-6 and IL-8 which were not inhibited by HC. Recombinant IL-1 but not anti ICAM-1 and anti LFA-3 induced GM-CSF and LIF. HC inhibited the secretion of GM-CSF and LIF induced by IL-1. None of the stimulants augmented the constitutive secretion of fibronectin or thymosin alpha 1 and HC inhibited thymosin alpha 1 secretion. TEC secretion of cytokines but not thymosin alpha 1 and fibronectin appear to be regulated in a more complex manner than heretofore recognized.