Treatment of rats with a single intravenous injection of 0.5 mg Sephadex G200 caused a blood and lung eosinophilia first detected on day 2, peaking on day 7 and declining to control levels by days 28 and 84 respectively. The increase in eosinophil numbers was accompanied by the development of pulmonary granulomas reaching their maximum size 2-7 days after Sephadex treatment and persisting for 84 days. This reaction was characterised by an early infiltration with polymorphonuclear leucocytes followed by increased numbers of mononuclear cells. Few eosinophils were associated with the Sephadex particles but large numbers were found in the adventitial layers of the lung. There was an increase in the size of the granulomas in rats given Sephadex on days 0, 2 and 5 compared with the response in rats given a single injection of Sephadex. The inflammation was more intense and more eosinophils were detected in the blood, alveolar tissues and adventitia. The effects of drugs given before each injection of Sephadex were evaluated. Dexamethasone, dapsone and isoprenaline reduced the blood and tissue eosinophilia but had no effect on the number of granulomas. Dexamethasone and dapsone, but not isoprenaline caused a slight reduction in the size of the inflammatory reaction around the Sephadex particles. Indomethacin had no effect on the granuloma formation, nor did it reduce the eosinophilia. Isoprenaline and indomethacin increased the proportion of eosinophils associated with the granuloma, although in neither instance did this reach significance.