Copper(II) and zinc(II) complexes of cephalexin have been prepared and characterized by microanalysis and by thermogravimetric, magnetic and spectroscopic analysis. The complexes were found to be five-coordinate, monohydrate, and ML2 type. The electron paramagnetic resonance spectral lines revealed rhombic distortion from axial symmetry, with g(parallel) > g(perpendicular) > g(e), in the elongated-tetragonal copper(II) complex. The geometry of the zinc(II) complex seems to be square-pyramidal. On complexation with copper and zinc the antimicrobial activity of cephalexin improved significantly. The copper complex was found to be active against kaolin paw oedema whereas the parent drug was inactive. These results suggest that the metallic elements should be seriously considered during drug design, and that complexes already reported should be subjected to clinical evaluation. Their use could provide an easy way of improving the activity and reducing the toxicity of drug substances.