In hospitals oil-based ink is occasionally used to write the patient's name, his room number, and names of the drugs that are used in a mixture on the plastic containers used for infusion. This is done to prevent medication errors. In the current study, the effect of an oil-based ink on the infusion fluid was investigated. Using an oil-bused ink containing xylene, letters and other characters were written on an infusion fluid container that was made of Polypropylene・Polyethylene・Ethylene vinyl acetate. It was confirmed that some xylene had been transported to the interior of the container (air phase). Furthermore, the xylene concentration in the air phase rose with repetitive writing and erasing, using the same oil-based ink. But no xylene was dissolved in the solution that was in the container (Iiquid phase). This observation convinced us that it is highly unlikely that the xylene in the ink can be infused directly into a patient's body. However, to assure the quality of the medication that is in liquid form, it is not desirable for a substance that does not normally exist as a component of the medication to move through its container. In the present experiment, physiological saline was used as a solution (liquid phase). Our findings showed that xylene had a high possibility of dissolving in solutions known for high xylene solubility (e.g., solutions with a large amount of amino acids or surfactants). Therefore, it is necessary to design a method to prevent ink from centering a container when an oil-based ink is applied to the outside of it.