Tri-tetracycline complexes of the invention can be identified by chromatography. Minocycline was subjected to thin layer chromatography on a Kieselgel G thin layer plate in a 50:10:10:10:10:10 solvent mixture containing benzene, methanol, ethanol, ethyl acetate, dimethylformamide and dimethylacetamide. Rf of minocycline:0.16, whereas the Rf value of the complex of minocycline formed with N-tris(hydroxymethyl)methylglycine is 0.63. Chelocardin was identified in a similar way, its Rf value in the previous system is 0.21, whereas the Rf value of the complex of chelocardin formed with N-tris(hydroxymethyl)methylglycine is 0.42. The Rf value of minocycline hydrochloride on Kieselgel G thin layer plate in a 60:20:10:10 solvent system of benzene methanol, ethyl acetate and dimethylformamide is 0.1, whereas the Rf value of the complex of minocycline with tris(hydroxymethyl)aminomethane is 0.2.
