A new series of transition metal complexes of Cu(II), Ni(II), Co(II) and Zn(II) have been synthesized from the Schiff base (L) derived from 4-aminoantipyrine, salicylaldehyde and o-phenylenediamine. The structural features have been arrived from their elemental analyses, magnetic susceptibility, molar conductivity, FAB-mass, IR, UV-Vis., 1 H NMR and ESR spectral studies. The data show that the complexes have composition of ML type. The UV-Vis., magnetic susceptibility and ESR spectral data of the complexes suggest a square-planar geometry around the central metal ion. The antimicrobial activities of the metal chelates against the bacteria Salmonella typhi, Pseudomonas aeruginosa, Bacillus subtilis, Escherichia coli and fungi Aspergillus niger, Aspergillus flavus and Rhizoctonia bataicola have been carried out. A comparative study of MIC values of the Schiff base and its metal complexes indicates that the metal complexes exhibit higher antibacterial activity than the free ligand. The nuclease activity of the above metal complexes shows only the copper complex cleaves DNA.