Chronic toxicity of indium arsenide (InAs) and indium phosphide (InP) was studied in male Syrian golden hamsters which received InAs or InP particles containing a total dose of 7.5 mg of arsenic or phosphorus by intratracheal instillations once a week for 15 weeks. As a control, hamsters were treated with the vehicle, phosphate buffer solution. During their total life span, the cumulative body weight gain of hamsters in the InAs group was suppressed significantly compared with that in the control group, but not in the InP group when compared with that in the control group. Concerning the histopathological findings of the lung, the incidence rates of proteinosis-like lesions, alveolar or bronchiolar cell hyperplasia, pneumonia, emphysema and metaplastic ossification observed in the InAs or InP group were significantly higher than those observed in the control group. From these results, it would seem that InAs and InP produced severe damage to the lungs of hamsters.