Bloom syndrome protein forms an oligomeric ring structure and belongs to a group of DNA helicases showing extensive homology to the Escherichia coli DNA helicase RecQ, a suppressor of illegitimate recombination. After over-production in E. coli , we have purified the RecQ core of BLM consisting of the DEAH, RecQ-Ct and HRDC domains (amino acid residues 642–1290). The BLM 642–1290 fragment could function as a DNA-stimulated ATPase and as a DNA helicase, displaying the same substrate specificity as the full-size protein. Gel-filtration experiments revealed that BLM 642–1290 exists as a monomer both in solution and in its single-stranded DNA-bound form, even in the presence of Mg 2+ and ATPγS. Rates of ATP hydrolysis and DNA unwinding by BLM 642–1290 showed a hyperbolic dependence on ATP concentration, excluding a co-operative interaction between ATP-binding sites. Using a λ Spi − assay, we have found that the BLM 642–1290 fragment is able to partially substitute for the RecQ helicase in suppressing illegitimate recombination in E. coli . A deletion of 182 C-terminal amino acid residues of BLM 642–1290, including the HRDC domain, resulted in helicase and single-stranded DNA-binding defects, whereas kinetic parameters for ATP hydrolysis of this mutant were close to the BLM 642–1290 values. This confirms the prediction that the HRDC domain serves as an auxiliary DNA-binding domain. Mutations at several conserved residues within the RecQ-Ct domain of BLM reduced ATPase and helicase activities severely as well as single-stranded DNA-binding of the enzyme. Together, these data define a minimal helicase domain of BLM and demonstrate its ability to act as a suppressor of illegitimate recombination.