The ternary complexes of lanthanum (III) with 2, 3-dimethyl-1-phenyl-4-salicylidene-3-pyrazolin-5-one (HL) and Ltryptophan, L-phenylalanine, L-isoleucine, L-proline and L-serine have been studied. The mixed ligand complexes have been prepared and characterized on the basis of elemental analysis, conductivity data, magnetic susceptibility measurements, UV-visible, infrared spectra, 1H NMR spectra and thermal analysis data. In these complexes, the Schiff base acts as a primary ligand and amino acids acts as secondary ligand which coordinates through the carboxylate oxygen and the amino nitrogen. The thermal stability of the complexes was studied and the weight losses were correlated with structure of the complexes. La (III) complexes were screened for their antimicrobial activities and show the potent biological activities.