The chemistry of the technetium-sulfur colloid produced by the reaction of sodium thiosulfate with acid was investigated. A commercial kit was duplicated, and analyses of elemental sulfur, bisulfite and residual thiosulfate were carried out. The colloidal dispersions were filtered through Nuclepore graded membranes, and the percentages of sulfur and of 99mTc in the various filtrates were determined. In all cases--with varying acid, thiosulfate and time of incubation--there was a rough agreement between the two percentages for particles 0.4 micron in diameter or more. However, for small particles (less than 0.1 micron) there was virtually no sulfur, but there was an appreciable percentage of technetium. It was concluded that the technetium sulfide nuclei formed first, and that the supersaturated sulfur deposited in part on them and in part on its own nuclei. It was found that raising the pH of the preparation to weakly alkaline values and reheating the solution dissolved most of the deposited sulfur by the reaction with sulfite to form thiosulfate, leaving much smaller, virtually sulfur-free technetium sulfide particles. Such a preparation was found to be as efficient as the technetium-antimony sulfide colloid for lymphograms in dogs. Potassium trithionate, K2S3O6, used in place of sodium thiosulfate, produced small Tc-S colloid particles with less sulfur than the conventional thiosulfate-acid system.