A unique class of radiopharmaceuticals labeled with the /sup 117m/Sn nuclide has been developed. The attractive properties of /sup 117m/Sn include a 14-day physical half-life and the emission of a single ..gamma..-photon with an optimal energy of 159 keV. A specialized apparatus was designed for the conversion of metallic /sup 117m/Sn to /sup 117m/SnCl/sub 4/ which was subsequently converted to a variety of useful /sup 117m/Sn-labeled organotin intermediates. The availability of this unique approach will now make possible the synthesis of a wide variety of /sup 117m/Sn-labeled compounds of biological interest. Such tissue-specific /sup 117m/Sn-labeled agents may represent a new class of useful radiopharmaceuticals. Continuing studies involving the preparation and testing of radiopharmaceuticals labeled with /sup 11/C, /sup 195m/Pt, and /sup 75/Se are also described. Several /sup 11/C-labeled amino acids including /sup 11/C-DL-tryptophan, /sup 11/C-1-aminocyclobutanecarboxylic acid (ACBC), and /sup 11/C-1-aminocyclopentanecarboxylic acid (ACPC) were prepared, and patient studies have demonstrated ACBC to be superior to ACPC for tumor localization. Studies with /sup 195m/Pt have recently been directed toward attempting to prepare high specific activity /sup 195m/Pt by the Szilard--Chalmers process. Progress is also reported in the development of an in vivo diffusion chamber assay technique that is being used to investigate …