Extracellular ECP, MPO, HIS, and PA-1 accumulate during storage of nonfiltered red-cell components, but the accumulation can be prevented by prestorage leukofiltration. In addition, bacterial antigens appear to induce significant release of the substances from white cells and platelets. Addition of supernatants from stored, nonfiltered WB and SAGM blood may increase the substance levels in a storage-time-dependent manner, and prestorage leukofiltration may prevent further increase by supernatants, except for MPO.