1. An intratracheal injection of murine recombinant interleukin 5 (mrIL-5, 2-15 microgram/0.25 ml/animal) induced a dose-dependent increase in the number of macrophages, eosinophils, neutrophils and epithelial cells in the bronchoalveolar lavage fluid (BALF) of guinea-pigs 24 h after administration. Bovine serum albumin (15 micrograms/0.25 ml/animal), used as a reference material, did not cause any change of this type. 2. The intratracheal administration of mrIL-5 at a dose of 15 microgram showed a tendency to increase the number of these pulmonary inflammatory cells and epithelial cells in the BALF at 12 h with a significant increase observed at 24 h. 3. Prednisolone (20 mg kg-1, i.p.) inhibited the mrIL-5-induced increase in macrophages, eosinophils, neutrophils and epithelial cells. Ketotifen (2 mg kg-1, i.p.) reduced the mrIL-5-induced increase in the eosinophil, neutrophil and epithelial cell populations. The simultaneous injection of 2% disodium cromoglycate (DSCG) into the trachea prevented the mrIL-5-induced increase in the number of airway epithelial cells, without affecting changes in the other inflammatory leukocytes. 4. These results suggest that mrIL-5 is a potent inducer of lung inflammation, in terms of increased inflammatory leukocytes and epithelial cells in guinea-pig BALF. Prednisolone, DSCG and ketotifen are effective against mrIL-5-induced pulmonary inflammation, especially the desquamation of bronchial epithelial cells.