Intravenous injection of Sephadex G200 particles into rats on days 0, 2 and 5 caused an increase in eosinophil numbers in blood and lung tissues. Peak numbers were obtained on days 3–12 and thereafter declined to approach control values by day 21. The rise in eosinophil numbers was paralleled by an increase in lung cell fragility as measured by a transient reduction in the number of viable cells isolated from parenchymatous tissue following the digestion of lung fragments in vitro. This decrease in lung cell viability was not seen in rats given a single injection of Sephadex. Dexamethasone, dapsone and isoprenaline given before each injection of Sephadex reduced lung and blood eosinophil numbers and prevented lung cell death. Aspirin and indomethaein were without effect. Incubation of normal lung tissues with disrupted peritoneal eosinophils reduced the numbers of viable cells recovered. No such effects were seen using intact eosinophils and disrupted or intact neutrophils and mononuclear cells. This system provides a model of lung cell damage associated with eosinophil infiltration in vivo.