Abstract: This invention relates to the inhibition of cell proliferation in mammalian host organisms. More particularly the invention relates to the utilization of selenodithiol compositions, particularly selenodiglutathione (GSSeSG) within a narrow concentration range, in which it is both effective as an inhibitor of cancer tumor cell proliferation in mammals, particularly in the prevention of neoplastic cell division in humans, while substantially non toxic outside this narrow range to the treated host organism. The invention also relates to a method for stimulating cellular proliferation at lower concentration of selenodiglutathione.