High mobility group (HMG) 1 protein is a highly abundant and an evolutionarily conserved chromosomal protein with two homologous DNA-binding domains (HMG boxes), A and B, attached by a short basic region to an acidic C-terminal tail. The protein has been implicated in a number of fundamental biological processes including DNA replication, transcription, recombination and repair. We demonstrate that HMG1 is able to enhance cohesive-end and blunt-end DNA ligation by T4 DNA ligase via its B domain. The C-terminal flanking sequence of the B domain (seven basic residues out of approximately 18) and a number of conserved amino-acid residues within the HMG box (mainly basic or hydrophobic) are required for efficient stimulation of ligation. Pull-down assays, electron and scanning force microscopy revealed that HMG1 can associate two DNA molecules via their ends even in the absence of complementary overhangs. We propose that HMG1 protein may be involved in the rejoining of DNA breaks by different DNA ligases due to its ability to bring DNA duplexes and their termini into a close proximity while leaving the ends accessible for ligation.