Type I restriction-modification (R-M) systems are comprised of two multi-subunit enzymes with complementary functions: the methyltransferase (similar to 160 kDa), responsible for methylation of DNA, and the restriction endonuclease (similar to 400 kDa), responsible for DNA cleavage. Both enzymes share a number of subunits, including HsdM. Characterisation of either enzyme first requires the expression and purification of its constituent subunits, before reconstitution of the multisubunit complex. Previously, purification of the HsdM protein had proved problematic, due to the length of time required for the purification and its susceptibility to degradation. A new protocol was therefore developed to decrease the length of time required to purify the HsdM protein and thus prevent degradation. Finally, we show that the HsdM subunit exhibits a concentration dependent monomer-dimer equilibrium. (C) 2012 Elsevier Inc. All rights reserved.