Abstract: A method of making a radiopharmaceutical composition by mixing an aqueous solution of pertechnetate with a reducing agent comprising tin metal or stannous ion to reduce the pertechnetate and a complexing agent to form a complex with the reduced technetium, is characterized by incorporating nitrate and/or nitrite in an amount to diminish oxidation of Sn.sup.2+ to Sn.sup.4+, and hence to prevent re-oxidation of technetium to pertechnetate, during preparation and storage of the complex. Nitrate and nitrite are superior to organic antioxidants previously proposed.A reagent for the purpose comprises the reducing agent, the complexing agent and the nitrate and/or nitrite usually in a sterile freeze-dried state.