l-Methionine enhances the toxicity of TeO 3 − and SeO 3 − to E. coli wild type. d-Methionine is ineffective. Similar effects are obtained with the methionine-requiring mutant A.T.C.C. 9663. The enhancement of TeO 3 − toxicity can be produced by a wide variety of sulfur compounds. Such compounds include those which serve as complete or partial sources of sulfur for E. coli in addition to those not metabolized by the organism. All effective compounds contain sulfur in its most reduced state (S −). SeO 3 tt- does not show this effect. The enhancement of TeO 3 − toxicity in contrast to SeO 3 − by more diverse sulfur compounds demonstrates that a sulfur-TeO 3 − interaction takes place which has an effect on E. coli that is not comparable to any SeO 3 −-sulfur interaction.