Binding of 45Ca2+ to nonhistone protein HMG1 was detected after fixation of the protein to nitrocellulose membrane. The same experiment with HMG1 peptides, derived from HMG1 by protease V8 digestion, allowed to identify the highly glutamic and aspartic C-terminal domain of HMG1 as a 45Ca2(+)-binding region. Measurements of 32P-labeled DNA retention on nitrocellulose filters revealed that in the absence of Ca2+, the affinity of HMG1 for linear DNA decreased upon an increase of pH from 7 to 8.4. However, when Ca2+ was included in the assay buffer, the affinity of HMG1 for DNA remained unchanged between pH 7 to 8.4 and was higher than in the absence of Ca2+. The effect of Ca2+ on HMG1 - DNA interaction was no longer observed upon removal of the C-terminal domain from HMG1.