Summary. The red cells of 22 patients with paroxysmal nocturnal haemoglobinuria have been evaluated by the complement lysis sensitivity test of Rosse & Dacie. Four patterns of abnormal susceptibility to complement lysis were seen. In Group 1 (10 patients), markedly abnormal complement-sensitive cells were present and cells of a second population were not distinguishable from normal cells. In Group 2 (four patients), markedly abnormal complement-sensitive cells were present and the second population of cells was clearly distinguishable, but not markedly different from normal. In Group 3 (five patients), three populations of cells were present: a markedly abnormal complement-sensitive population, a population of moderately abnormal cells and a population of normal-appearing cells. In Group 4 (three patients), no cells with marked complement sensitivity were seen; two populations were present, one was apparently normal and the other moderately abnormal (3–4 times as sensitive as normal). During the period of observation two patients in Group 4 acquired a markedly abnormal complement-sensitive population.
The proportion of cells in each population varied during the course of the illness. In three patients the proportion of complement-sensitive cells increased during the onset of the illness, and in two patients the proportion decreased gradually during the resolution of the disease. Increase in the total production of red cells resulted in an increased proportion of complement-sensitive cells, as did decrease in haemolytic rate. Increase in the haemolytic rate resulted in a decreased proportion of complement-sensitive cells.