Recent reports demonstrating that the lethal effects of radiation on tumour cells can be augmented by tumour necrosis factor-alpha (TNF-alpha) prompted us to investigate whether this premise holds true for the LS174T human colon adenocarcinoma cell line. Three different techniques were used to assess cell damage: 3H-thymidine (3H-TdR) uptake, clonogenic survival, and vital dye exclusion. In these assays human recombinant TNF-alpha treatment was administered before single-dose-gamma-radiation at 4, 6, 8, or 10 Gy. Oxygen radical formation by the tumour cells in the presence of TNF-alpha and radiation, alone and in combination, was also investigated. TNF-alpha and radiation, when used as single modalities, decreased LS174T cell viability with time. However, treatment with TNF-alpha before irradiation resulted in highly significant reductions in 3H-TdR uptake and decreased clonogenic survival compared to their counterparts receiving only radiation. Our data show that these two measurements of tumour-cell damage correlate well. No difference was noted in vital dye exclusion when comparisons were made between TNF-alpha+radiation and radiation alone. This latter finding may be partly due to the fact that although apoptotic cells are 'dead', they generally do not become more permeable to normally excluded macromolecules. Chemiluminescence measurements indicate that the radiation-enhancing mechanism of TNF-alpha may be related to oxygen radical production by the LS174T cells. Taken together our results suggest that TNF-alpha may be useful as an adjunctive modality in the radiotherapy of colon cancer.