Early, dose-dependent and transient alterations were observed in the plasma membranes of human blood cells after X-irradiation in vitro as detected by a concanavalin A-binding technique. Platelets, lymphocytes and erythrocytes, separated in a one-step procedure from a single blood sample, showed different radiosensitivities within the dose range of 0.1--5 Gy. The complex evaluation of the surface alterations of blood cells might indicate the range of the absorbed dose.