Summary. The red cells in paroxysmal nocturnal haemoglobinuria (PNH) are of three sorts: population I in which the cells appear to be normal, population II in which the cells are 3–5 times more sensitive to complement (C) lysis than normal and population III in which the cells are 10–15 times more sensitive to complement. In all previously reported patients, two or three populations were present. Two patients were found to have only cells of population II. These cells are like those of population I (normal cells) and population III in fixation of antibody and the first and fourth components of complement. The cells of populations II and III fix larger amounts of the third component than normal. The amount of lysis obtained for a given amount of fixed C3 is much less for cells of population II than for cells of population III.