The electrophoretic mobility of erythrocytes from 3 patients with PNH was measured by standard techniques. Erythrocyte mobility is a measure of the net negative surface charge of these cells. In each of the 3 patients studied in mobility was significantly increased over normal cells indicating that there is an increase in negative surface charge of PNH erythrocytes. Following the Ham's and sucrose lysis tests; surviving cells showed mean mobilities that were not statistically different from normal cells. This suggests that these cells with higher surface charge are vulnerable to complement lysis.