Interleukin-6 (IL-6) is a multifunctional cytokine which plays a role in the stimulation, inhibition, differentiation, and regulation of cell growth. IL-6 has been shown to act as an autocrine growth factor in several tumors, and is expressed by a variety of tumors. IL-6 also exhibits a regulatory role in the hypothalamo-pituitary axis. It is produced by both the hypothalamus and the pituitary, and it induces the secretion of anterior pituitary hormones. Because of the regulatory role of IL-6 in tumor growth and its involvement in the pituitary, we decided to evaluate IL-6 and IL-6 receptor expression in pituitary tumors. For this purpose we utilized complimentary cDNA probes specific for the IL-6 and IL-6 receptor mRNA, as well as monoclonal anti-IL-6 antibodies for immunohistochemical analysis. Our results show that the IL-6 gene is expressed in the normal pituitary tissue. However, the non-functioning and functioning pituitary tumors such as the prolactin and growth hormone secreting tumors express increased levels of the IL-6 gene. The IL-6 receptor gene was only expressed in the prolactin secreting and non-functioning pituitary tumors. These results show that the IL-6 and IL-6 receptor gene expression is enhanced in pituitary tumors, thus suggesting a possible role of IL-6 in the pathogenesis of these tumors.