Terramycin and Achromycin form chelate compounds with various metals, especially stable compounds with zirconium and thorium, as previously reported with Aureomycin. Their molar ratio is 1:1 in thorium chelates and 1:2 in zirconium chelates. Further investigations on the chelate formation of Apoterramycin and Isoaureomycin showed that phenolic β-diketone in tetracycline structure was very important as the chelating group, as was indicated in the previous paper. It is also suggested that the chelation of tetracylines must further involve the 12-1 bond.