The acute toxicity of incinerator fly ash was studied in guinea- pigs. Fly ash collected from a municipal incinerator was analyzed for cadmium (7440439), lead (7439921), mercury (7439976), zinc (7440666), and carbon (7440440). Eight Hartley-guinea-pigs were exposed to the fly ash at concentrations of 314 milligrams per cubic meter in a specially designed inhalation chamber 6 hours/day for 5 consecutive days. Before and after each exposure and at selected times up to 50 days post exposure, tidal volume (VT) and respiration frequency were measured before or after carbon-dioxide challenge. The guinea-pigs were killed on day 50 post exposure. The mean concentrations of cadmium, lead, mercury, zinc, and carbon in the fly ash sample were 477, 2134, 25, and 14301 parts per million and 7.34%, respectively. The concentrations of cadmium, lead, zinc, and mercury in the lungs from the exposed animals were significantly elevated. VT was not significantly by flyash exposure. Respiratory frequency was decreased. Both VT and respiratory frequency in fly ash exposed animals were decreased by carbon-dioxide challenge. Both parameters tended to recover to the control values during the post exposure period. Moderate to severe pneumoconiosis characterized by an interstitial macrophage reaction, thickening of alveolar septa, and moderate constriction of the airways accompanied by smooth muscle hypertrophy were seen in fly ash exposed guinea- pigs. The authors conclude that the tested fly ash is not very potent for inducing an acute pulmonary effect. The delayed histopathological changes seen are suggestive of possible chronic obstructive lung disease.