The effect of Mn

++

and DMSO on the sequence recognition of the restrictionendonuclease Hind Ⅲ was investigated.Under the normal reaction condition of10mM Tris-HCl

pH8.0

10mM MgCl

2

60mM NaCl and 7mM 2-ME

there is onlyone cutting site of Hind Ⅲ on pBR322 and only one band appears on gel electro-phoresis.But multiple bands can be seen on electrophoresis gel when Mg

++

isreplaced by Mn

++

or when DMSO is added.The DNA fragments generated inthe Hind Ⅲ-DMSO reaction are converted to linear DNA molecules.In theseconditions

the cellular DNA of the host（Haemophilus influenzac）is alsodigested by Hind Ⅲ.These data show that the specificity of cleavage of HindⅢ is relaxed and cleavages occur at many other sites in the presence of Mn

++

and DMSO.