AM1 semiempirical molecular orbital calculations have been used to probe the complexation sites for naked and hydrated magnesium ions to the different conformations and protonation states of tetracycline. The calculations reveal a wealth of possible magnesium complexation sites within a small energy range, but also indicate that magnesium complexation does not change the conformational behavior of tetracycline significantly. A hitherto unknown solvated conformation is suggested for deprotonated tetracycline.