The purpose of this study was to assess the toxicity of 2 organic fiber-types. The study has two components . First, one study compared the pulmonary effects of inhaled para aramid (p-aramid) vs. size-separated aerosolized chrysotile asbestos fibers in rats. In another study, the effects of inhaled cellulose in rats were investigated. For both studies, the parameters assessed were clearance/biopersistence of inhaled p-aramid or cellulose RFP relative to asbestos fibers, as well as cell proliferation in the lungs of rats following 2-week exposures. Rats were exposed nose-only to asbestos at concentrations of 459 and 782 f/cc, or to p-aramid RFP at 419 or 772 f/cc, or to an aerosol of Thermocell mechanical wood pulp (Laxa Bruks AB, Rofors, Sweden) cellulose fibers for 2 weeks at target concentrations of 300 and 575 fibers/cc. Following exposures, the lungs of rats were evaluated at several times postexposure. Lung clearance/biopersistence studies in p-aramid-exposed rats showed biodegradation of inhaled p-aramid, as evidence by breakage and progressive shortening of inhaled fibrils (with increased residence time in the lung). In contrast, clearance of short chrysotile fibers was rapid, but slow or insignificant for long chrysotile fibers, as evidenced by a progressive increase over time in the mean lengths of fibers recovered from the lungs of exposed rats. Cellulose RFP were also cleared slowly and appeared to be more biopersistent than the p-aramid RFP. Two week, high-dose exposures to p-aramid or cellulose in rats produced transient increases in pulmonary cell proliferation parameters. In contrast, asbestos exposure produced sustained cell proliferative responses. These results demonstrate differences in the pulmonary responses to these two inhaled organic respirable fiber-types when compared to chrysotile asbestos.