A number of smoke toxicity screening tests are being used, and some have been proposed for regulatory purposes. These methods differ markedly in the combustion device, the combustion conditions, the exposure conditions and even the test animal. However, virtually all use an "LC50" value, or some lethality determination, as the principal or sole measure of toxicity. In this study, lethal potencies were determined by the NBS Cup Furnace, the Radiant Furnace and the UPITT test methods for one nylon-and four PVC-based materials. These results were correlated with analytical data to identify the major toxicants responsible for the lethalities. This study shows that, while all of these methods have deficiencies, the UPITT method has more serious limitations. The use of this method to determinate "LC50" values for screening materials may lead to erroneous assessments of the toxic hazards of these materials to humans.