Thyroid follicular cells (TFC) are a common target of autoimmune attack, but the role they play in inciting and maintaining this attack is unclear. TFC express cytokines, adhesion molecules, and class I and II major histocompatibility complex molecules, but without additional signals that costimulate T cells, they may down-regulate, rather than stimulate, T cell function. In this report, we have investigated whether TFC can express the CD40 molecule, which plays a crucial role in the reciprocal two-way communication between T and B cells. We have shown by immunohistochemistry and flow cytometry that CD40 is expressed by TFC in vivo and in vitro in both autoimmune and nonautoimmune glands. CD40 expression was up-regulated by interleukin-1alpha and interferon-gamma, but not by TSH. Although there was no significant effect of CD40 ligation on cAMP synthesis or [3H]thymidine incorporation, there was a significant increase in interleukin-6 release by TFC. Thus, although TFC do not express members of the B7 family of T cell costimulators, they do express CD40, indicating the possibility of mutually stimulatory T cell-TFC interaction. This has important implications, both for TFC synthesis of immunological mediators and for the biasing of T cell behavior toward a T helper 2-type phenotype.