The ektacytometer, a device to measure erythrocyte flexibility, has been used to evaluate antisickling agents that covalently modify hemoglobin S (HbS). The instrument has been adapted to produce a continuous gradient of oxygen pressure in the measuring cuvette, which permitted the rapid determination of sickle cell rigidity over the complete oxygenation range. Inspection of curves allows classification of the compounds according to their mode of action: altering oxygen affinity or increasing deoxy-HbS solubility. Reagents that modify amino groups, thiols, and histidine, as well as a crosslinking agent, were examined. The method directly evaluates deoxygenated cell deformability rather than cell shape. Many of the compounds that are effective in preventing the morphological sickling of deoxygenated sickle cells do not necessarily restore cell deformability. The method also readily detects membrane damage brought about by covalent agents that nonspecifically derivatize membrane proteins. Cystamine and pyridoxal appear to improve deformability in deoxygenated SS cells at concentrations that do not damage the membrane. This method, which examines the intact cell, fills a gap in the available experimental techniques for drug evaluation, between studies of isolated hemoglobin and in vivo studies.