The DNA methyltransferase component of the type I restriction and modification enzyme of Escherichia coli K12 has been purified. The active component, a trimer of molecular mass 170 kDa consisting of one DNA recognition subunit (S) and two modification subunits (M), showed the expected preference for modifying a hemimethylated substrate rather than an unmethylated one. Small amounts of the dimers M2 and M1S1 were also isolated. Subunit rearrangements of the three protein species occurred on ion exchange and heparin-agarose chromatography. Denaturation of the trimer gave folding intermediates, and these and the dimer forms isolated during purification may reflect the assembly of the protein in vivo. Enzyme activity was recovered on refolding the denatured protein by dilution of the denaturant. A comparison of the predicted isoelectric points of all known S subunits of type I restriction and modification enzymes revealed values that correlated with the arrangement of type I systems in several families. Electrostatic interactions may explain the different subunit stoichiometries observed during purification of type I enzymes and the differing preferences for hemimethylated DNA displayed by the three type I families.