The sulphydryl population on erythrocyte membrane is shown to vary as a function of the nutritional status of the cell. When an assay based on reaction with Ellman's reagent and controlled conditions were used the mean values (SEM) for the sulphydryl population on the membranes of normal erythrocytes incubated overnight in the presence and absence of glucose were found to be 3.29 (0.27) and 2.56 (0.25) million sulphydryl functions per cell respectively. Under identical conditions rheumatoid erythrocytes incubated in the presence and absence of glucose were found to have a significantly lower sulphydryl population--1.54 (0.08) and 1.15 (0.08) million respectively. The predominant concentrations of sulphydryl groups on the membrane are found at sites on the transmembrane proteins and, in particular, on the hexose transport protein. By influencing the nutritional status of the cells significant differences in activity between the normal and diseased state have been identified and these may have a role in the aetiology of rheumatoid arthritis by altering the response of cells to oxidative stress.