We have studied three glycosylphosphatidylinositol (GPI) linked proteins on the erythrocytes of 14 patients with paroxysmal nocturnal haemoglobinuria (PNH). The pattern observed was bimodal in 12 of the patients and trimodal in two. Ten patients had a red cell population with normal CD59 antigen (membrane inhibitor of reactive lysis, MIRL), decay accelerating factor (DAF or CD55) and lymphocyte function-associated antigen (LFA-3 or CD58) and a second abnormal PNH population with absent CD59 antigen, DAF and LFA-3. The other two patients with a bimodal pattern had a red cell population with normal CD59 antigen, DAF and LFA-3 and an abnormal population with reduced, but not absent, CD59 antigen and DAF. The LFA-3 on the abnormal red cells in these two patients appeared to be only slightly reduced. The two patients with a trimodal pattern had a normal population, a population with reduced, not absent, CD59 antigen and DAF, and a population with complete absence of CD59 antigen, DAF and LFA-3. The accuracy of the Ham test in estimating the proportion of red cells with the PNH defect in the two types of PNH was assessed. The case of one patient who appeared to be 'rescued' from severe aplastic anaemia by the development of PNH is described.