To clarify the instillation toxicity of low level of indium phosphide (InP), 0, 1.2, 6.0 and 62.0 micrograms/kg body weight of InP particles were instilled intratracheally in male Fischer 344 rats, and the effects of InP were examined on the following day (day 1) and on the 8th day (day 8) after instillation. Indium was measured but not detected in the serum, liver, kidney, spleen, thymus and brain. Dose-related mild elevation of superoxide dismutase (SOD) activity and lactate dehydrogenase (LDH) activity in bronchoalveolar lavage fluid (BALF) were found on day 1 without increases of inflammatory cells and total protein (TP) in BALF, which suggested the response of neutrophils and alveolar macrophages to instilled InP, and/or the manifestation of a very early stage of inflammation. Only in the 62.0 micrograms/kg-instilled group on day 8, were neutrophils, lymphocytes, TP, LDH, total phospholipid and total cholesterol in BALF increased, and desquamation of alveolar epithelial cells and amorphous exudate in alveolar lumen observed by histopathological examination. These results suggested that InP caused pulmonary inflammation and epithelial cell damage up to 8 days following instillation dose of 62.0 micrograms/kg, but that its effect was considered irrelevant at instillation doses of 6.0 micrograms/kg or below in rat.