In contrast with previously published observations, this study has found no increase in the net negative surface charge of the red blood cells (RBC) from patients with various forms of polycythaemia when compared with normal subjects. Two sub-populations of RBCs with different electrophoretic mobilities (EPM), termed fast and slow, were present in samples from both patients and normal individuals. Frequently, the slow cell population in RBCs from patients with polycythaemia, particularly the apparent polycythaemia sub-group, was of a lower EPM and contained more cells than the corresponding population in normal subjects. The membrane-bound sialic acid content of RBCs from patients with primary proliferative polycythaemia, associated with iron deficient RBC changes, was increased if the results were expressed per unit volume of RBCs and decreased if the sialic acid concentration was presented per number of RBCs. These results imply that differences in surface charge of RBCs are of no value in discriminating between primary and secondary polycythaemia.