This inhalation toxicity study in rats was conducted to assess the hazard potential for workers inhaling Nylon respirable fibers. Groups of 48 male rats each were exposed, nose-only, 6h per day, 5 days per week, for 4 weeks to aerosols of uncoated, finish-free Nylon respirable-sized, fiber-shaped particulates (RFP) at concentrations of 0, 4, 15 and 57 fibers (f)/cm3. Nylon RFPs were prepared using flock rotary cutters followed by vigorous opening procedures. After exposures, the lungs of sham and Nylon-exposed rats were assessed at 1 and 8 days, and 1, 3, 6 and 12 months postexposure. The results showed that the retained mean lung burdens at 1 day postexposure were 1.75E+07 RFP/lung (high level). Mean lengths and diameters of the Nylon aerosol were 9.8 and 1.6 microm, respectively. Lung clearance of Nylon RFPs was rapid over the 12-month period. There were no significant increases in lung weights, indications of pulmonary inflammation, or alveolar macrophage functional deficits in Nylon-exposed animals versus controls based on cell differentials, bronchoalveolar lavage (BAL) fluid analyses, and macrophage phagocytosis or chemotaxis activity. Histopathology revealed no adverse lower pulmonary or upper respiratory effects. In summary, the no-observed-effect level (NOEL) for inhaled Nylon RFP was 57f/cm3 (20mg/m3), the highest concentration tested.