Members of the SNF2/SWI2 family, characterized with sequence motifs similar to those found in DNA and RNA helicases, play roles in various aspects of cellular fundamental processes such as transcriptional regulation, chromosome stability, nucleotide excision repair, and recombination. We have isolated a novel member of the human SNF2/SWI2 family, RAD54B, which is highly homologous to mammalian RAD54. The RAD54 gene is a member of the RAD52 epistasis group which is involved in the recombinational repair of DNA damage. Here we demonstrate that human Rad54B (hRad54B), like human Rad54 (hRad54), associates with human Rad51 (hRad51). Both hRad54B and hRad54 associate with hRad51 through their NH(2)-terminal domains, but there are differences in their ways of association with hRad51. In contrast to Rad54, whose association with Rad51 is induced by ionizing radiation, Rad54B associates with Rad51 constitutively in immunoprecipitation experiments. Also, the failure to detect the interaction between hRad54B and hRad51 in the yeast two-hybrid assay suggests that their interaction, unlike that between hRad54 and hRad51, may be indirect. Immunofluorescence microscopy revealed that hRad54B formed nuclear foci that colocalized with hRad51, hRad54, and BRCA1. These findings suggest that Rad54B may be functionally distinct from Rad54, although it may play an active role in recombination processes in concert with other members of the RAD52 epistasis group.