The plasma concentration of calprotectin was measured before, during and after apheresis in patients with Guillain-Barré Syndrome (GBS), Waldenstrøm's syndrome or hypercholesterolaemia and in healthy donors of platelets. Increased calprotectin levels were found after plasma exchange in the Waldenstrøm's syndrome patients, probably caused by release of the protein from activated leukocytes. The decreased calprotectin values observed in the other patients, may be due to plasma dilution. Unexpectedly, the GBS patients were found to have high initial calprotectin levels in plasma but not in cerebrospinal fluid. In donors, normal and unchanged calprotectin concentrations were found throughout.