was noted in the mmvf21 group minimal interstitial fibrosis was observed that gradually decreased after the 15 and 3 month nonexposure periods in this study the pathological changes found in the lungs of exposed rats were in accordance with the pathology previously reported from full lifespan inhalation studies this may indicate that for fibres belonging to the manmade vitreous fibres group a well conducted biopersistence study is sufficient to predict possible pathogenic effects for new fibre types the biological parameters examined in a 90 day study may indicate little additional information to contribute to the prediction of the outcome of carcinogenicity studies