Dehydroepiandrosterone (DHEA), a major adrenal secretory product in men and women, is a potent inhibitor of mammalian glucose-6-phosphate dehydrogenase (G6PDH). Long-term treatment with this steroid has previously been found to suppress spontaneous breast cancer development in C3H mice. DHEA is now shown to inhibit Epstein-Barr virus (EBV)-induced morphologic transformation and stimulation of DNA synthesis in human lymphocytes. 16 alpha-Br-epiandrosterone, a DHEA analog that is about 60 times as potent as DHEA as an inhibitor of G6PDH, is much more effective as an inhibitor of EBV-induced transformation.