Previous in vitro studies using the exposure of suspensions of normal human erythrocytes to mercury vapour (Hg0) in Warburg vessels revealed a broad variation of the cellular Hg uptake in the presence of hydrogen peroxide and furnished evidence for an oxidation of Hg0 by the catalase-H2O2 pathway. This report describes the uptake of Hg0 in the same exposure system as it depends on haematocrit, H2O2 supplementation and exposure time. The results indicate that the uptake of vapour by the whole suspensions was virtually independent of haematocrit and of the H2O2 supplementation at high rates of H2O2 infusion, whereas at low rates it was directly proportional to the amount of peroxide added. Since the retention of mercury by the medium is negligible, the haemoglobin-related (specific) uptake is inversely proportional to the haematocrit at high rates of H2O2 infusion. These results are consistent with the conclusion that the incorporation of Hg0 by erythrocytes is controlled by two processes: transfer of vapour to the cells and enzymic oxidation. The net rate of uptake is limited by the oxidizing capacity at low, and by the vapour transfer at high H2O2 supplementation. As a practical consequence of these findings the spread of the specific uptakes reported in the literature is considerably reduced if the values observed when transfer becomes rate limiting are related to the haematocrit.