Fungal manganese peroxidase was found to convert the persistent chemical warfare agent adamsite (phenarsarzin chloride) in a cell-free reaction mixture containing sodium malonate, Mn2+ ions, and reduced glutathione. The organo-arsenical compound disappeared completely within 48 h accompanied by the formation of a more polar metabolite with a clearly modified UV spectrum. Thus, As(III) in the adamsite molecule was oxidized by manganese peroxidase to As(V) which added dioxygen and released chloride.