For the preparation of most 99m Tc radiopharmaceuticals, SnCl 2 has remained the agent of choice for reduction of Tc 7+ to lower valency states, which facilitates its chelation by compounds of diagnostic importance. We have developed a simple technique in which SnCl 2 lyophilized in a glass vial, either alone or on a solid matrix of polymeric microspheres (beads), was used. Tin-113 (t 1/2 -115 d) was used as a tracer, which facilitated quantitative assessment of loss or release of tin in the reaction mixtures. The feasibility and efficacy of this technique were examined for preparations of four 99m Tc-labeled peptides, in which SnCl 2 was used as a reducing agent for radiolabeling, a procedure well established in our laboratory. Labeling efficiencies for all four peptides using SnCl 2 on solid phase was greater than 95%, as compared to less than 90% (P ΒΌ 0:05) for SnCl 2 lyophilized without the solid matrix. Colloid formation was less than 3% in either case. The stability of SnCl 2 was greater than six months for solid matrix, and less for that without the microspheres. The 113 Sn measured as a daughter product 113m In indicated that release of SnCl 2 from microspheres in reaction mixture was 85 AE 3%, as compared to 80 AE 5% lyophilized alone. The recovery of 99m Tc in solution from microspheres was 95-100%. The large size of the microspheres used (649 mm) eliminated the risk of drawing them through