Composting has been advocated and is being used as an economical method for remediating 2,4,6-trinitrotoluene (TNT)-contaminated soils. However, evidence suggests that TNT is transformed into products of unknown toxicity during the process. This study was undertaken to examine the in vitro cytotoxicity and mutagenicity of TNT and several of its degradation products/metabolites. TNT was equally cytotoxic to H4IIE cells and Chinese hamster ovary-K1 (CHO) cells (LC50 of 4 g/ml vs 24 microg/ml, with overlapping 95% prediction intervals), indicating that TNT does not need to be metabolized to exhibit cytotoxicity. Four metabolites studied, 4-hydroxylamino-2,6-dinitrotoluene; 2-amino-4,6-dinitrotoluene; 4,4',6,6'-tetranitro-2,2'-azoxytoluene; and 2,2',6,6'-tetranitro-4, 4'-azoxytoluene, were equally cytotoxic to both H4IIE and CHO cells. The LC50s were in the 3- to 18-microg/ml range and were not significantly different from TNT cytotoxicity in both cell lines. 4-Amino-2,6-dinitrotoluene (4-A) was moderately less cytotoxic than TNT to H4IIE cells, but was noncytotoxic to CHO cells. This result indicates that 4-A is metabolized to a cytotoxic compound. Both TNT and its metabolites exhibited only slight mutagenicity at high doses in one or both of the mutagenicity assays. While composting may reduce the levels of TNT in composted material, the hazard associated with TNT-contaminated soils is probably lower, but still uncertain.