Potentiometric equilibrium measurements have been performed at (25.0 +/- 0.1) degrees C and ionic strength 1 = 0.1 mol.dm(-1) KNO(3) for the interaction of the biologically important ligands N-acetylhistidine, N-acetyl L-leucine, N-acetylglutamic acid, N-acetylhistamine, N-acetylaspartic acid, and La(III), Gd(III), Sm(III), Tb(III), Eu(III), and Dy(III) with the nucleotides guanosine 5'-monophosphate (5'-GMP), cytedine 5'-monophosphate (5'-CMP), inosine 5'-monophosphate (5'-IMP), adenosine 5'-monophosphate (5'-AMP), adenosine 5'-diphosphate (5'-ADP), adenosine 5'-triphosphate (5'-ATP), and cytedine 5'-triphosphate (5'-CTP) in 1:1:1 and 1:2:1 ratios. The formation constants of various mixed ligand complexes were inferred from the potentiometric titration curves. Initial estimates of the formation constants of the resulting species and the formation constants of the different N-acetyl amino acid (NAA) and nucleotide (NU) complexes in metal ligand ratios 1:1 and 2:1 have been refined with the SUPERQUAD computer program. Confirmation of the formation of binary and ternary complexes of the type Ln(III)-NU-NAA and the possible recognition of the purine and pyrimidine motionucleotides by N-acetyl amino acid complexes of lanthanides in aqueous media have been carried out using UV Spectroscopic and fluorimetric measurements.