The catalytic and physical Áchemical properties of organophosphorus hydrolase (OPH) modified by the addition of an N-terminal dodecahistidine tag (His 12 -OPH) have been investigated. Introduction of the His 12 -tag caused a 30-and 74-fold increase in catalytic efficiency of the enzyme with parathion and methyl parathion, respectively, compared to OPH. Concurrently, the His 12 -OPH had a more alkaline pH-optimum and extended temperature range than OPH and OPH modified with a hexahistidine tag. A study of His 12 -OPH thermostability showed that the enzyme had a tendency to oligomerise. This resulted in a decrease in the enzymatic activity of His 12 -OPH at temperatures B/508C, but provided the enzyme with much higher thermostability at temperatures !/508C, compared to OPH.