DETERMINATION of the diagnostic value of N,N′-bis (thiazole)-2,7-fluorene-disulphonamide-35S (TFDS-35) revealed some differences in its distribution in the tissues of rats bearing Walker carcinosarcoma as compared with tumour-free animals, regarding the TFDS-35 level in red blood cells, blood plasma, liver, spleen, kidneys and urine1. Furthermore, the tumour tissue showed a relatively high concentration of the disulphonamide. The present investigation was aimed at explaining these differences by histological and autoradiographic examination of selected organs and tumour tissue of rats.