Investigations were carried out to evaluate the level of incorporation of radiolabeled 2,4,6-trinitrotoluene (TNT) and metabolites into the bacterial biomass of two different bacterial species after cometabolically mediated TNT transformation. Biotransformation experiments with C-14-TNT indicated that TNT was not mineralized; however, carbon derived from TNT became associated with the cells. It was found that more than 42% of the initially applied radiolabel was associated with the cell biomass after cometabolic C-14-TNT transformation with the strictly anerobic Desulfovibrio species strain SHV, whereas with the strictly aerobic Serratia plymuthica species strain B7, 32% of cell-associated C-14 activity was measured. The remainder of the radiolabel was present in the supernatants of the liquid cultures in the form of different TNT metabolites. Under anoxic conditions with the Desulfovibrio species, TNT was ultimately transformed to 2,4,6-triaminotoluene (TAT) and both diaminonitrotoluene isomers, whereas under oxic conditions with the Serratia species, TNT was converted to hydroxylaminodinitrotoluenes and aminodinitrotoluenes, with 4-amino-2,6-dinitrotoluene (4ADNT) being the major end product. In both culture supernatants, small amounts of very polar, radiolabeled, but unidentified metabolites were detected. At the end of the experiments approximately 92% and 96% of the originally applied radioactivity was recovered in the studies with the Serratia and Desulfovibrio species, respectively.