Hb composition in varying populations of red blood cells as well as the oxygen-transport function of hemolysates and blood were studied in patients with paroxysmal nocturnal hemoglobinurea (PNH). The dissociation curves obtained for oxyhemoglobin of blood and hemolysates were diphase, that denoted heterogeneity of Hb content and appearance of Hb with high O2 affinity in the blood of PNH patients. Isoelectrofocusing permitted detection of two additional fractions X1 and X2. The immunochemical method was used to show that X1 fraction was semi-Hb, and X2 fraction, by its physicochemical properties, was affined to Hb conformer--Hb precursor during its biosynthesis. The data obtained have shown that in PNH patients a pathologic population of red blood cells with incomplete process of hemoglobin formation appears in the blood flow.