Anhydrotetracycline (AHTC) is the major toxic decomposition product of the antibiotic tetracycline. The complexation of AHTC to Mg(II), Al(III), and Fe(III) was studied in aqueous medium using absorption and circular dichroism measurements. The study of the Mg(II)-AHTC interactions at pH 7 indicated the formation of the MHL and M2L species in which an Mg(II) ion is coordinated to the C11 and C12 oxygens of the BCD ring system. In the M2L species, a second metal ion coordinates to the N4 and O3 positions on ring A, inducing the ligand to adopt the "twisted" conformation. At pH 4, an MHL species is formed with Al(III) by complexation of the metal ion to O11 and O12. At pH 1, Fe(III) forms an MH2L species, probably by coordination of the metal to 012 and 01. The stability constants of all species were calculated. The possible participation of Mg(II) in the mechanism of toxicity of tetracycline is suggested.