Synthetic vitreous fibers are in widespread use but the parameters that dictate their carcinogenicity are still a matter of scientific debate. The free radical activities of a panel comprising an asbestos sample and five different respirable synthetic vitreous fiber samples were determined, to address the hypothesis that carcinogenic fibers have greater free radical activity than noncarcinogenic fibers. On the basis of recent inhalation studies, the six samples were divided into three carcinogenic fibers-amphibole asbestos, silicon carbide, and refractory ceramic fiber 1 (designated with the abbreviation RCF 1)-and three noncarcinogenic fibers-man-made vitreous fiber 10 (a glass fiber sample designated with the abbreviation MMVF 10), Code 100/475 glass fiber, and RCF4. All experiments were carried out with equal fiber numbers. Of the two assays of free radical activity used, the plasmid assay of DNA scission showed only amosite asbestos to have free radical activity, while the salicylate assay of hydroxyl activity showed that both amosite asbestos and RCF1 release hydroxyl radicals; silicon carbide fibers had no free radical activity in either of the assays. None of the noncarcinogenic fibers demonstrated free radical activity in either of the assays. The differences in the two assays in demonstrating free radical activity with RCF1 may be due to increased release of Fe from RCF1 under the more acid conditions of the salicylate assay, which was confirmed by the fact that soluble iron caused hydroxylation of salicylate. Presence of an iron chelator inhibited the ability of the RCF1 fibers to cause hydroxylation of salicylate, demonstrating that RCF1 generates hydroxyl radical by Fenton chemical reaction in the same way as amphibole asbestos.