Silicone tubing is frequently used for gas exchange in cell culture systems, due to its biocompatibility and high permeability to CO₂ and O₂. In cell culture chambers, medium pH and oxygen levels are often maintained by gas exchange through a coil of silicone tubing. Culture medium is recirculated between the gas exchanger and the culture chamber which contains a suspension of cells. We report that the type of agent used for silicone vulcanization (peroxide or platinum) can markedly affect its biocompatibility, and that tobacco cell culture represents a particularly sensitive indicator of tubing cytotoxicity. Under the conditions studied (cell suspension maintained with forward–reverse flow and stirring), peroxide-cured silicone tubing was toxic to the tobacco BY2 cell culture, in contrast to the platinum-cured silicone tubing that was completely biocompatible. Upon further investigation by mass spectrometry, it was determined that a component with a molecular mass of 288 Da, possibly a tetrachlorinated biphenyl, was present in culture medium in contact with peroxide-cured tubing but not in medium in contact with platinum-cured tubing. Additional curing of peroxide-cured tubing resulted in cell morphology and viability comparable to controls. These data suggest that improperly cured silicone tubing can release catalytic byproducts which can be toxic to plant cells, and that the BY2 tobacco cells represent a suitable model system for studies of materials biocompatibility.