Two novel mineral fibres, ‘Saffil’* alumina and ‘Saffil’ zirconia were tested in cell culture and in animals to assess their cytotoxicity and tissue reactions. The toxicity of these two materials to rat peritoneal macrophages in cell culture was low in comparison with chrysotile asbestos, suggesting that, in animals, they would be unlikely to cause progressive fibrosis. When the two types of Saffil fibre were injected into the peritoneal cavity of animals, it was observed that there was a difference in the reactions. Saffil alumina caused the deposition of connective tissue containing a higher proportion of collagen than that around Saffil zirconia. Saffil zirconia had been eliminated from more animals than Saffil alumina and was seen more frequently in lungs and lymph nodes. These observations are probably related. The intra peritoneal reaction to chrysotile asbestos was typical and demonstrably different from either Saffil fibre.