Previous experiments in our laboratory suggested the existence of an excreted biomolecule from Methanosarcina thermophila, capable of rapid carbon tetrachloride (CT) and chloroform (CF) transformation. Batch experiments with filtered supernatants from M. thermophila cultured in the presence and absence of elemental iron (Fe0) were performed to determine if such a molecule existed. Filtering the supernatant with a 0.22 μm filter did not alter the CT or CF transformation rate or pattern. Supernatants from cultures grown in medium without Fe0 were able to facilitate rapid CT transformation, but could not catalyze subsequent CF transformation. However, when the organisms were cultured in the presence of high concentrations of Fe0 (10 g/L), the filtered supernatant catalyzed rapid CT and CF transformation. Autoclaving the supernatant did not significantly alter the CT transformation rate or pattern; however, heat treatment decreased CF transformation. The extracellular factor appeared to have a high transformation capacity for CT; upon dechlorinating 82 μm CT, the rate of transformation did not decrease.