Rare-earth elements are emerging contaminants of soil and water bodies which destiny in the environment and effects on organisms is modulated by their interactions with natural ligands produced by bacteria, fungi and plants. Within this framework, coordination by harzianic acid (HL), a secondary metabolite, of a selection of tripositive rare-earth cations Ln (Ln = La, Nd, Sm and Gd) was investigated at 25 °C, and in a CHOH/0.1 M NaClO (50/50 ) solvent, using mass spectrometry, circular dichroism, UV-Vis spectrophotometry, and pH measurements. Experimental data can be satisfactorily explained by assuming, for all investigated cations, the formation of a mono-complex (LnL) and a bis-complex (LnL). Differences were found between the formation constants of complexes of different Ln cations, which can be correlated with ionic radius. Since gadolinium is the element that raises the most concern among lanthanide elements, its effects on organisms at different levels of biological organization were explored, in the presence and absence of harzianic acid. Results of ecotoxicological tests suggest that harzianic acid can decrease gadolinium biotoxicity, presumably because of complex formation with Gd.