The sequence of the 224 residues of HMG 1 suggests it consists of three domains. We have previously proposed Cary et al. (1980 Eur. J. Biochem. 131, 367–374 that the A and B domains can fold autonomously and that there is also a small N domain. Several proteases are now found to cut at the end of the B domain (at or close to residue 184). It is shown that the A+B‐domain fragment also folds and probably contains all the helix of intact HMG 1. The stability of the B domain is enhanced by the presence of the A domain. The acidic C domain undergoes a coil → helix transition on lowering the pH. Several peptides have been prepared by cleavage at tryptophan. Peptide 57–C‐terminus contains complete B and C domains but does not fold. In the absence of the A domain the C domain is thus able to destabilise the B domain. It is concluded that the stability of the B domain in HMG 1 is due to interaction with the A domain and the C domain has a separate function from the other domains.