Equilibrium study of complex formation of M2+ ions (M = Co, Ni, Cu and Zn) with ampicillin (AH) and aminoacids (B), viz. glycinate, α-alaninate, β-alaninate and aspartate in ternary systems in aqueous solution at 37°C at a fixed ionic strength I = 0-1 moldm-3 (NaNO3) indicated the formation of mixed-ligand complexes of the types: M(A)(B), M(A-H)(B), M(A-H)(B)(OH), in addition to the binary M(A) and M(B) complexes. Stability of the mixed-ligand complexes were correlated with the modes of coordination of the ligands.