AQ55 is a high molecular weight, water-dispersible, amorphous polyester used in applications where the exclusion of solvents and conventional surfactants is desirable, such as water-based adhesives, coatings, emulsions, paint primers, cosmetics and detergents. Potential health effects were evaluated in rats exposed by inhalation for about 13 wk to mean concentrations of 0, 2.4, 19.6 or 199 mg/m3 AQ55 polymer. No mortality occurred and body weights were unaffected. Mean relative liver weights in all treated male groups were slightly higher than control weights, but were not judged to be treatment related. Absolute liver weights and all other organ weights except lung weights were normal. Haematology, clinical chemistries and gross pathology were unremarkable. Exposure-related changes in the 199 mg/m3 groups included increased mean absolute and relative lung weights, accumulations of macrophages and acute inflammatory cells in alveolar and bronchial lumina, and increased numbers of macrophages in sinusoids of peribronchial lymph nodes. Minor accumulation of macrophages in alveolar lumina was the only exposure-related change in the 19.6 mg/m3 group. No exposure-related effects were seen in the 2.4 mg/m3 group. AQ55 produced no systemic toxicity, and aerosols of AQ55 do not appear to be toxic to pulmonary tissues following subchronic inhalation exposure.