Submerged electrical discharges between copper-containing electrodes rendered the treated liquid microbicidal. Part of this activity was unstable and decreased rapidly during the first few minutes. It might have been caused by cuprous ions or substances with oxidative activity. The stable microbicidal activity was due to copper released from the electrodes. This copper existed only partly in ionic form. Inorganic salts and most organic substances tested decreased the bactericidal effect of discharge-treated water. Such substances also diminished the protracted killing effect which was observed when bacteria suspended in water of relatively high purity were subjected to transient electric arcs.