In the process of direct labelling of proteins with 188Re, the influence of Sn (II) in the concentration range of 5 x 10 (-4)-l 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 approximately 1 microgram (10 (-8) M) Sn/mg protein in gluconate, 16% of the protein thiol groups were bound to Sn, and at approximately 2.4 micrograms (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.