We have studied the mechanism of tumour uptake of 201Tl by in vivo and in vitro studies. In a series of patients with breast cancer (n = 26), lung cancer (n = 56) and lymphoma (n = 15), the time course of tumour uptake of 201Tl paralleled that in the myocardium with almost identical times of peak uptake being obtained in tumours and myocardium. In a patient with hepatic metastases from colonic cancer undergoing laparotomy, 99mTc labelled microspheres and 201Tl were injected into the hepatic artery and biopsies of metastatic and normal liver tissue obtained. The tumour to normal liver activity ratios for 201Tl were one tenth of those for 99mTc microspheres. In the final part of the study, cells from a lung cancer tissue culture line were incubated for 30 min with 201Tl with and without the addition of cardiac glycoside, which acts a sodium potassium pump blocker. The cells exposed to the cardiac glycoside showed markedly decreased uptake of 201Tl compared to the cells not so exposed (0.6% +/- 0.1% vs 11.8 +/- 0.7.2% of the administered dose). The mechanism of 201Tl uptake of tumours is similar to that in the myocardium. Sodium potassium pump activity appears to be more important than tumour blood flow. 201Tl uptake may provide a useful means of studying tumour viability.