Fibres of various defined length distributions were tested in vitro and in vivo. Tumour cells with phagocytic ability were used as the in vitro test system. DNA and protein synthesis and cell proliferation and enzyme release (LDH) were determined. Using gravimetrical and numerical doses in cultures, good indications of the toxicity of single fibres were found. It was possible to demonstrate clear-cut relationships between fibre length and cytotoxicity. In this test system, the toxicity of fibres increases with increasing length. The results of the cell experiments also furnish important indications of the doses of fibrous dusts to be used in animal tests and of the possibility of comparing different fibres with each other, especially with respect to the numerical or gravimetrical quantification of fibre fractions. Amosite, chrysotile, crocidolite and glass fibres of three or four different length spectra were tested.