Treatment of human erythrocytes with dithiothreitol (DTT) increases the sensitivity of normal cells to complement (C)-mediated lysis. We have investigated the mechanism through which DTT increases cell susceptibility to complement by comparing the interactions of complement proteins with DTT-treated erythrocytes and with normal cells. In addition, we have studied the effect of DTT on the physical state of the erythrocyte membrane. Results indicated that the DTT primarily affects the interactions of the late components of complement with the cell membrane. In particular, the insertion efficiency of C9 and its ability to form tubular poly-C9 are enhanced on DTT-treated cells. Electron spin resonance (ESR) spectroscopic analyses of the treated and untreated membranes showed essentially no correlation between bulk membrane fluidity and the DTT-induced change in lytic susceptibility, suggesting no gross disruption of the membrane lipid structure by DTT. In view of the fact that DTT-treated erythrocytes have been proposed as a possible model for the abnormally complement-sensitive erythrocytes from patients with paroxysmal nocturnal hemoglobinuria (PNH) which are deficient in a 75,000 mol. wt membrane protein called decay accelerating factor (DAF), we explored the possibility that DAF might be affected by DTT. Studies with anti-DAF F(ab')2 antibodies indicated that DAF activity is protected from DTT-treatment. These results are reinforced by the observation that DTT-treatment of DAF-deficient Type III PNH-E also led to enhanced lysis of PNH-E, implying that DTT affects membrane structures other than DAF. Thus, we conclude: (1) that DTT increases the lytic susceptibility of human erythrocytes to late components of human complement by modifying membrane structures to facilitate C9 insertion and polymerization, and (2) that DTT-treated erythrocytes are not a suitable model for PNH erythrocytes.