Medullary thyroid cancer (MTC) is a relatively uncommon neuroendocrine tumor that arises from the calcitonin-secreting parafollicular cells of the thyroid gland. Unfortunately, MTC frequently metastasizes, precluding curative surgical resection and causing significant morbidity. Thus, there is an urgent need for new treatment modalities. Tautomycin and tautomycetin are antifungal antibiotics isolated from Streptomyces spiroverticillatus and Streptomyces griseochromogens, respectively. Glycogen synthase kinase-3beta is a serine/threonine protein kinase that regulates multiple cellular processes and is important in various cancers, including MTC. Treatment with tautomycin and tautomycetin decreased neuroendocrine markers, suppressed hormonal secretion, and inhibited growth through apoptosis in MTC cells. Importantly, we describe a novel action of these compounds: inhibition of glycogen synthase kinase-3beta.