Type II restriction and modification enzymes are an ideal model system for studying specific DNA-protein interaction because of their small size, simple catalytic requirements for activity, and sequence specificity. We chose to study the interaction of the Type II endonuclease and its corresponding methylase from Agrobacterium tumefaciens SSI with DNA. The specific methylase for the recognition site of AtuS I endonuclease from Agrobacterium tumefaciens SSI was purified and its molecular and catalytic properties were studied. For the purification of the enzyme, 20 g of cells were used and disrupted by French press at 20,000 p.s.i.. After ammonium sulfate fractionation, the enzyme was further purified by phophphocellulose column chromatography and DEAE-cellulose column chromatography. The methylase has shown to have the site-specific methylation activity on the AtuS I recognition sequence. ## And AtuS I methylase prefers double-stranded DNA than singlestranded DNA as a substrate. The enzyme showedmaximum activity at values between 7.0 and 8.0 at 37$^\circ$C. and does not essentially require NaCl, $Mg\bar{\cdot}^+$. AtuS I methylation did not inhibit subsequent restriction activities of BstN I and Zan I endonucleases.