The evidence presented shows that both methyl methacrylate and ethyl methacrylate monomers can diffuse through the rubber bulb of a dropper dispenser-style bottle. Methyl methacrylate is substantially more efficient in this regard than is ethyl methacrylate, which leads to the loss of these products into the environment immediately around the dropper bottle. If a dropper bottle of these agents is stored in a confined space such as a storage tub along with certain local anesthetic cartridges, the monomer vapor can enter the cartridge and contaminate its contents. A combination of methyl methacrylate in the presence of 2% lidocaine with 1:100,000 epinephrine appears to be the most reactive of the combinations tested. Local anesthetic cartridges should not be stored in a confined space with dropper bottles containing either methyl methacrylate or ethyl methacrylate monomers.