(Kasprzak, Malejka, Gabryel, 1965). The tumour concentration for this conipound appeared, however, not to be highly selective: the ratios of the radioactivity concentration in tumour to that in liver or spleen or blood plasma were lower than 0 5. These findings favour the hypothesis (Argus, 1961) that the presence of free acidic groups in the fluorene disulphonamido high molecular compounds contributes to their binding at the cellular adsorption sites by basic proteins.The other aspect of the diagnostic studies with the fluorene disulphonamido group was to observe the reaction of the reticulo-endothelial system in the presence of tumour. The phenomenon of an impaired phagocytic function of the reticuloendothelial system was demonstrated in the presence of various tumours in different species using N,N'-bis(naphthalene)-2, 7-fluorene[35S]disulphonamide (Argus and Hewson, 1954; Argus, Hudson, Seepe, Kane and Ray, 1962). The same reaction was observed with N,N'-bis(p-sulphamoyl-phenyl)-2,7-fluorene[35S]disulphonamide (Malejka, 1962). The importance of the precise chemical structure is shown by the failure of N,N'-bis(thiazole)-2, 7-fluorene[35S]disulphonamide to localize in liver and spleen of tumour-free animals to a greater extent than in tumour-bearers (Malejka, 1965).The present investigation was aimed at studying the influence of various substituents in the fluorene disulphonamide molecule on the distribution of the compounds in tumour-bearing and tumour-free rats. Five new derivatives of 35S-labelled 2,7-disulphonamido-fluorene were synthesized in which the substituents on the nitrogens were chosen so as to permit study of the effect of different structural types on the tissue localization.