This paper summarizes the biological effects of refractory ceramic fibres (RCFs). RCFs are aluminosilicate glass insulation wools with similar chemical properties to other synthetic vitreous fibres (SVFs) or ‘man-made vitreous fibres ’ (MMVFs). There is concern that RCFs could be significantly more pathogenic than other SVFs. This paper critically reviews the data on which this perception is based. Morbidity studies on workers in RCF manufacturing indicated that, in the United states, RCF exposure was associated with an increased incidence of pleural plaques and in both the united states and Europe with statistically significant changes in some measures of lung function (though not at present exposure levels). No interstitial fibrosis was found. An ongoing mortality study of limited statistical power has failed to indicate any increased incidence of lung cancer or mesothelioma. Findings in several early animal studies led to a large series of inhalation studies where rats exposed to high levels of RCF developed fibrosis and tumours but not those exposed to other SVFs. Similarly hamsters exposed to one sample (RCF1) developed mesothelioma. Subsequent analyses of the data indicated that the RCF used in these experiments had a significantly greater proportion of non-fibrous particles than those present in the other types of SVFs tested or in workplace air. Short-term studies indicated that