The uptake of 201Tl-chloride in tumours has recently been investigated in several authors' laboratories. We are reporting studies on the uptake in malignant melanoma in mice. Generally, we confirm the published data qualitatively in finding higher uptake in tumour than in muscles. We confirm the rapid blood clearance and high kidney concentration, but we did not measure the myocardial uptake. We observed the concentrations in kidney muscle and tumour to rise during the first four hours while that in the liver fell from one hour onwards. We did not observe markedly higher uptake in melanin rich tumour than in other tumours.