Chromosome condensation occurs in mitosis before the separation of sister chromatids, and requires DNA topoisomerase II and a group of proteins called SMCs. The resulting condensed chromosomes in metaphase have a complex hierarchical structure. SMCs, the components of condensed chromosomes, are also required for the separation of sister chromatids and gene dosage compensation, and are found in a range of organisms from yeasts to mammals. However, the mechanisms by which the SMCs contribute to chromosome condensation are unknown. We have studied chromosomes in fission-yeast SMC mutants cut3-477 and cut14-208, which remain largely non-condensed during mitosis at the restrictive temperature (36 degrees C). To test their role in DNA condensation, we isolated the proteins Cut3 and Cut14 as an oligomeric complex, and tested their interactions with isolated DNA. The complex efficiently promoted the DNA renaturation reactions (the winding up of single-strand DNAs into double helical DNA) as much as approximately 70-fold more efficiently than RecA, which is a bacterial protein with similar activity. The activity of the mutant complex was heat sensitive. As DNA winding by renaturation is a potential cause of supercoiling, the SMC complex may be implicated in promoting the higher-order DNA coiling found in condensed chromosomes.