A Schiff-base, I, was prepared by the condensation reaction between the amine trimethoprim and the aldehyde triethyl orthoformate in 1:1 M ratio. The metal complexes of Cu2+, Zn2+ and La3+ were derived from the ligand I. Based on the elemental analysis and the IR, UV–Vis, 1H NMR and MS spectra analyses the chemical structures were emphasized. The γ-irradiation stability of the products was investigated via the post-radiation spectral analysis by means of UV–Vis techniques. Some radiolysis products were suggested with the guidance of the MS results. Also, the thermal stability of the Zn-complex was examined by the thermogravimetric analysis (TGA) which confirmed the establishment of coordination. Relatively low molar conductance values were observed for the Cu- and Zn-complexes suggesting the association of the anions in the coordination sphere and consequently the non-electrolytic behavior. The ionic structure of the La-complex was inferred to obey the ratio 1:2 electrolyte. The magnetic properties together with the UV–Vis reposted data revealed the octahedral coordination of the Zn- and La-complex and the tetragonal distortion geometry of the Cu-complex. The La-complexes revealed only moderate inhibition against the Gram-negative Escherichia coli, whereas the Zn-complex revealed significant inhibition. The latter showed also similar effect against the Gram-positive Bacillus subtilis and Streptococcus pyogens (ATCC-19615).