Abstract: A new formulation of Tc-99m colloid is described. The new colloid contains a high ratio of perrhenate to thiosulphate, cysteine, and a prepared higher final pH than found previously. In preparing the colloid preferred rhenium sulfide is not used as an ingredient. The pH of the final formulation and can be between about 5.5 an about 8.0, and the ratio of rhenium to sulfur is from about 0.05 to about 1.2. In addition, the new colloid has excellent radiochemical purity, and a much smaller particle size distribution than has generally been previously available for sulfur colloid preparations.