The additivity of catalyzed hydrogen evolution currents in solutions of hemoglobins (Hb), binary with respect to these compounds, was studied by d.c. polarography in ammonia buffer solutions containing Co(II). The solutions contained two different hemoglobins, each having different numbers of SH groups in the molecule. The composition of the hemoglobin mixture was changed, either keeping [Hb(1)] + [Hb(2)] constant, or [Hb(1)] + [Hb(2)] variable but [Hb(1)] constant. The experimental results were compared with the sum I-s of catalytic currents of the constituent Hb species, taken from their experimental catalytic current-hemoglobin concentration relationships. The catalytic currents were evaluated taking into account no preferential adsorption of any hemoglobin in the formation of I-s, or preferential adsorption of Hb(1) or Hb(2). In general, five types of additivity were proposed. Calculated total currents were compared with experimental data. It was proved that in all cases experimental currents were much lower than the I-s obtained assuming no preferential adsorption of any hemoglobin. This testifies to some interactions between constituent hemoglobins of the mixture in the adsorption layer, causing a considerable reduction of the total current. Results concerning tested types of additivity proved that experimental results are best approximated by the calculated catalytic currents, if preferential adsorption of the hemoglobin containing more SH groups is assumed. Some remarks regarding the practical application of catalytic current measurements to solutions containing mixtures of proteins are given.