The fibrogenicity of short crocidolite (12001284) and erionite (12510428) fibers was studied in rats. Fisher-344-rats were exposed to crocidolite or erionite preparations containing fibers longer than 6 microns or shorter than 5 microns by inhalation or intrapleural injection. Selected rats were killed after 3, 6, 12, or 24 months and the lungs were removed and examined for histopathological evidence of fibrosis, pleural mesothelioma, and bronchiolar alveolar hyperplasia (BAH). Lung tissue samples from animals killed at 24 months were analyzed for their fiber content. After intrapleural injection at least 90% of the rats exposed to long fibers of either crocidolite or erionite developed mesothelioma. When the short fiber preparation was used crocidolite induced mesothelioma in only one rat. Short erionite fibers did not cause any mesotheliomas. After inhalation exposure more than 90% of the rats exposed to the long fiber preparation of erionite developed mesotheliomas. The long fiber preparation of crocidolite caused only a single mesothelioma. The short fiber preparations of erionite or crocidolite did not induce mesotheliomas. The long fiber preparations of erionite and crocidolite induced two cases of BAH. Inhalation exposure of either long erionite or crocidolite fibers caused minimal fibrosis. The short fiber preparations produced only a tissue reaction. Lung tissue sections contained significant numbers of long erionite or crocidolite fibers, but almost no short fibers. The author concludes that animals treated with dusts containing short and long fibers tend to selectively retain the longer fibers.