[en] The labelling of plasma proteins with technetium-99 m (^99m Tc) was analysed using plasma and plasma with red blood cells. Trichloroacetic acid precipitation showed that labelling efficiency of plasma proteins was slightly larger than when plasma was alone for stannous ion (Sn ⁺⁺) concentration in which the labelling of red blood cells was the highest. The cellulose acetate electrophoresis technique used, showed that the labelling of the albumin fraction with ^99m Tc increased when the Sn⁺⁺ concentration decreased. For the gamma globulin fraction it was observed that the labelling increased with the increase of Sn⁺⁺ concentration. (M.A.C.)