1. The results obtained in this study confirm the previous ones related to the presence in Nereis (both in control and contaminated animals) of a low molecular weight protein having the capacity to bind Cd (Dennaï et al., 1986). This component probably also binds Zn and Cu. 2. By SDS-PAGE and amino acid analysis, the molecular weight of the purified protein was estimated at about 10 kD. 3. However, results of molecular analysis using gel filtration on Superose 12 indicate that the Cd-binding protein called metalloprotein II (MP II) because of its occurrence in the second main protein peak after gel filtration on a Sephadex G-75 column has a molecular weight of about 20 kD in its native state. 4. It is suggested that the native protein is a dimer of two identical subunit polypeptides. 5. The present study establishes that this molecule which only contains 0.9% of Cys is clearly not related to metallothionein.