The aim of this study was to determine the efficacy of Tc-99m- glutathione (GSH) in scintigraphic demonstration of osteosarcoma tumour in mice and the effect of gamma irradiation of tumour on tumour uptake of Tc-99m-GSH. The biodistribution of 99mTc-GSH was studied in 30 Balb C mice 3 weeks after isotransplanting osteosarcoma OTS-64 in their thighs. The mice were injected with 400 mu Ci Of (TC)-T-99m-GSH in 0.1 mi through the tail vein. They were equally divided into two groups. Tn the second group the tumours were subjected to gamma irradiation for 10 min (20 Gy). The mice in both groups were killed at 1, 3 and 6 h. Scintigrams were obtained at each time point. The organs, tumours, some muscle and some blood were removed, weighed and assayed for radioactivity. Tumour, liver and muscle sections were also obtained for gross autoradiographic studies. The tumours were well visualized on scintigrams, The tumour uptake values as a function of time after injection were 3.27+/-0.80, 1.53+/-0.69, and 1.51+/-0.55 for the control and 5.18+/-1.28, 0.399+/-0.120, and 1.67+/-1.05%/g for the irradiated groups at 1, 3 and 6 h, respectively. The tumor-to-muscle concentration ratios were 34.03+/-12.2, 21.4+/-11.3 and 18.7+/-11.4 for the control and 18.8+/-7.2, 3.63+/-1.9, and 24.1+/-9.0 for the irradiated groups, respectively. The gross autoradiographic images of tumour sections indicated focal sites of increased uptake within tumour tissue, indicating the presence of necrotic areas. In conclusion, Tc-99m-GSH accumulated in osteosarcoma and resulted in high tumour-to-other tissue concentration ratios in mice. The increase in uptake values after tumour irradiation might be a result of increased demand of tumour cells for GSH attributable to its well-known biological function as a reducing agent in addition to increased blood flow and capillary permeability in malignant tissues.