The current EPA health effects test guidelines for chronic inhalation toxicity and/or carcinogenicity studies are not specific enough for the testing of fibrous substances which have been identified as priority substances for risk reduction and pollution prevention due to potential health concern. Thus, there is a need for EPA to develop standardized health effects test guidelines for fibrous substances that can be used by EPA to obtain the necessary toxicologic information for risk assessment. On July 28, 1999, EPA published the proposed combined chronic toxicity and carcinogenicity guidelines for testing fibrous particles. The proposed test guidelines were developed based on the guidance provided by an expert panel specific for the design and conduct of chronic inhalation studies of fibers in rodents.