Acute toxicity of chlorides of rare earth elements has been determined as EC50 for movement inhibition of oligochaeta Tubifex tubifex, EC50(T.t.). There is no statistically significant difference among EC50 values for the rare earth chlorides. Their QSAR is: EC50(T.t.) = 0.0781 ± 0.0058 (mol l⁻¹) (n=14) and log EC50(T.t.) = −1.107, corresponding to the fact that there is also no significant difference in the values of their physicochemical properties. The EC50(T.t.) values are comparable with those of, e.g., BaCl₂ or CdCl₂ classified as toxic compounds. Thus, the EC50(T.t.) indicates the hazard of rare earth salts for aqueous environment as well. The literature search shows that usage and waste of rare earth elements and their compounds are increasing. Several laboratory findings indicate their possible influence on calcium metabolism in organisms.