Interactions of cadmium (Cd) ions with bovine serum albumin (BSA), bovine hepatic metallothionein (MT), calf thymus histone and deoxyribonucleic acid (DNA), and bovine hepatic chromatins were studied in the presence and absence of divalent zinc (Zn), copper (Cu), mercury (Hg), or lead (Pb) ions, using equilibrium dialysis at pH 7 and at 37°C. The BSA had 3.5 Cd-binding sites with an apparent affinity constant of 1×10(5). The other metal ions inhibited the binding by reducing the affinity constant and the number of Cd-binding sites in BSA. There were 6 high affinity and 13 low affinity Cd-binding sites in the MT. Zinc ions had poor efficacy in reducing the binding of Cd to the MT. However, the Cu(2+) and Hg(2+) ions inhibited the Cd binding to a considerable extent, the former ions being more potent in this respect. Histone did not bind Cd. There were two kinds of Cd-binding sites in DNA: One mole of Cd per four moles DNA-phosphorus at low affinity sites, and one mole of Cd per 6.7 moles DNA-phosphorus at high affinity sites. Their apparent association constants were 8.3×10(5) and 4.4×10(6) M, respectively. The other metal ions had inhibitory effects on the binding of Cd to DNA. Histone reduced the Cd-DNA interactions to only a minor extent. The other metal ions reduced the binding of Cd to DNA-histone complex to a small extent. Cadmium binds to the euchromatin (Euch), heterochromatin (Het), and Euch-Het mixture almost equally. The other metal ions reduced the binding maximally in Euch-Het followed next in order by Het and Euch. Cupric ions were the most potent inhibitors of the interactions of Cd with the nuclear materials.