[en] In the process of direct labelling of proteins with ¹⁸⁸Re, the influence of Sn(II) in the concentration range of 5 x 10^-4-1 mg/mL of protein was studied using ^117mSn radiolabel in the presence of two transchelation buffers--sodium gluconate and sodium citrate. It was shown that Sn(II) readily binds to the thiol groups on the protein, and the fraction of Sn bound to the protein was 5 to 10 times higher in citrate than in gluconate for all Sn(II) concentrations studied. At saturation point of ∼1 μg (10^-8 M) Sn/mg protein in gluconate, 16% of the protein thiol groups were bound to Sn, and at ∼2.4 μg (2 x 10^-8 M) in citrate, 32% of thiols were bound to Sn. A mechanism was proposed for the involvement of Sn(II) in labelling of pre-reduced proteins with ¹⁸⁸Re via formation of protein-tin-¹⁸⁸Re(V) reaction intermediate. It was further shown that the amount of Sn(II) in reaction mixture must exceed a certain level in order to achieve high labelling yields, and this level of Sn(II) was found to be different for citrate and gluconate buffers