The carbocyanine dye 3,3'-dipropylthiodicarbocyanine iodide has been used to investigate changes in membrane potential (Em) which occur upon binding of complement proteins C5b-9 to the plasma membrane of blood platelets. Gel-filtered platelets exposed to C5b6 and C7 in serum-free medium show no change in Em from that of controls, as indicated by either 3,3,'-dipropylthiodicarbocyanine iodide fluorescence or by the distribution of [14C]tetraphenylphosphonium bromide. Addition of complement proteins C8 and C9 to the C5b67 platelets results in partial depolarization of Em, which spontaneously repolarizes to basal levels within 15-20 min at 37 degrees C. Under these conditions, C5b-9-treated platelets show no increase in lysis over complement-free controls. Isotonic replacement of external sodium by either potassium or choline alters both the rate and extent of membrane depolarization and inhibits the platelets' capacity to repolarize after C5b-9 assembly. Repolarization of Em to basal levels is also completely blocked by addition of ouabain, confirming that this recovery is mediated by the plasma membrane Na+/K+ pump. These results demonstrate that membrane binding of the C5b-9 proteins can induce a transient change in Em when bound to the plasma membrane at a sublytic concentration, providing a mechanism for target cell activation by these potentially cytolytic proteins.