A significant increase was found in the number of white blood cells, including neutrophils, in the peripheral blood of patients during plasma exchange. However, the spontaneous chemiluminescence of peripheral white blood cells (basically characterized the activated state of neutrophils) was not elevated. This finding suggests a relative deficiency in the metabolism of neutrophils during plasma exchange. This idea is supported by the observation that the levels of malonyldialdehyde in plasmas, after plasma exchange, do not increase, indicating that the activation dependent lipid peroxidation in neutrophils did not take place.