Many investigators have reported on the influence of various metal ions on antibiotics. The direct evidence for binding of metal ions with antibiotics is, however, very rare. The authors happened to find the precipitate formation by Mg⁺⁺ ion and aureomycin, and studied this phenomenon on various sorts of metal ions and antibiotics. The results are summarized as follows: 1) A marked yellow precipitate was formed by 10⁻² 10⁻³ M Mg⁺⁺ and 10⁻³-10⁻⁴M aureomycin. 2) A yellow precipitate was formed by 10⁻³M Mn⁺⁺ and 10⁻³-10⁻⁴M aureomycin, and was also formed by 10⁻⁴M Mn⁺⁺ and 10⁻³M aureomycin. 3) Just, like aureomycin, terramycin formed a yellow precipitate in the presence of Mg⁺⁺ or Mn⁺⁺. The degree of precipitate formation was, however, very weak as compared with aureomycin. 4) The precipitate formation between Mg⁺⁺ and aureomycin appeared at the pHs over 6.5 and became more marked with the rise of pH. 5) The acid solution of the yellow precipitate formed by Mg⁺⁺ and aureomycin showed a strong antibacterial action.