To investigate the relationship between antimicrobial activities and the formation constants of Cu-II, Ni-II and Co-II complexes with three Schiff bases, which were obtained by the condensation of 2-pyridinecarboxyaldehyde with DL-alanine, DL-valine and DL-phenylalanine, have been synthesized. Schiff bases and the complexes have been characterized on the basis of elemental analyses, magnetic moments (at ca. 25 degreesC), molar conductivity, thermal analyses and spectral (i.r., u.v., n.m.r.) studies. The i.r. spectra show that the ligands act in a monovalent bidentate fashion, depending on the metal salt used and the reaction pH = 9, 8 and 7 medium, for Cu-II, Ni-II and Co-II, respectively. Square-planar, tetrahedral and octahedral structures are proposed for Cu-II, Ni-II and Co-II, respectively. The protonation constants of the Schiff bases and stability constants of their ML-type complexes have been calculated potentiometrically in aqueous solution at 25 +/- 0.1 degreesC and at 0.1 M KCl ionic strength. Antimicrobial activities of the Schiff bases and the complexes were evaluated for three bacteria (Bacillus subtillis, Staphylococcus aureus, and Escherichia coli) and a yeast (Candida albicans). The structure-activity correlation in Schiff bases and their metal(II) complexes are discussed, based on the effect of their stability contants.