The complexation of 2-aminobenzothiazole (2abt) [A] with Ni(II) in presence of amino acids viz., glycine (gly), L-alanine (ala), L-valine (val) and L-phenylalanine (pal) [B] in 50% (v/v) water-ethanol mixture containing NaClO4 (0.15 M) has been studied by pH metrically at various temperatures (300, 310, 320 and 330 ± 0.1 K). Mixed ligand complexes of types NiAB and NiAB2 were observed and their stability constants were determined. The stabilization of mixed ligand complexes over binary analogues has been derived from Δ log K, log X' and log X values. The thermodynamic parameters ΔG, ΔH and ΔS were derived from the temperature dependence of the equilibrium constants. The complexation behavior has also been studied by means of electronic spectra. On the basis of stability constants and electronic spectra, it is revealed that the mixed ligand complexes have six-coordinated octahedral structure. The binary and mixed ligand complexes were screened for their microbial activities in vitro on common bacteria, fungi and yeast. The DNA cleaving activities were studied by electrophoresis method.