The following aspects have been investigated in 10 patients affected by Huntington's disease )HD): --extensive haematological investigations; --red cell enzyme activities and level of the most important glycolytic intermediate compounds; --protein, lipid and carbohydrate composition of the erythrocyte membrane and membrane polarity; --effects of in vitro aging on red cell membranes. Lack of 4.5 protein band in SDS-PAGE and 14-fold decrease in membrane-bound catalase were found in the in vitro aged red cells from the 10 HD patients examined. Na+ + K+ATPase was slightly higher than normal in all the patients. Red cells from 5 out of 8 patients showed a decrease in reduced glutathione and phosphoenolpyruvate levels and/or an increase in hexokinase, glucose-6-phosphate dehydrogenase, pyruvate kinase and glutathione reductase activities. The haematological investigations, the protein lipid and carbohydrate composition of the fresh red cells, the membrane polypeptide aggregates and the membrane polarity evaluated by microspectrofluorometric analysis were normal.