OBJECTIVE: To find whether continuous ceramic filaments (CCFs) and silicon carbide whiskers (SiCWs), which are used in many industries as reinforcing materials in advanced ceramic composites, are carcinogenic in the intrapleural inoculation assay. METHODS: Samples of SiCWs, CCF, International Union Against Cancer crocidolite, or saline were injected into the pleural cavities of female F344/N rats to find whether the samples of SiCW and CCF had the potential to induce mesotheliomas after the direct application of the materials to the surface of the pleural mesothelium. RESULTS: Rats injected with two of the three individual samples of SiCW or the crocidolite had significantly reduced life spans compared with the rats treated with saline, CCFs, or the third SiCW sample. Rats treated with either of the two SiCW samples or crocidolite developed mesotheliomas. By contrast, rats treated with saline or CCF did not. The two SiCW samples that induced shortened life spans also induced a higher rate of mesothelioma (87%-90%), than the crocidolite (57%) and the third SiCW sample (23%). CONCLUSION: SiCWs but not CCFs could induce mesotheliomas after intrapleural injection in rats. The difference in biological activity between the SiCW samples could not be explained on the basis of their physical dimensions or biological activity toward cultured cells. Results from this study indicated that SiCWs should be handled with care as they might be carcinogenic if inhaled. However, there is controversy as to whether results of intrapleural injection assays are sufficient to determine a fibre's carcinogenic activity. The results also showed that a collection of fibrous materials such as SiCWs could have considerably different biological activities despite similar physical dimensions.