Intratracheal injection of samples of naturally occurring and man-made mineral fibres into guinea pigs showed that while long fibre samples produced marked fibrosis, short fibre specimens produced only a macrophage reaction. In most cases the long fibre samples were administered in smaller doses than the short. The samples tested were crocidolite asbestos, a synthetic fluoramphibole and two specimens of glass fibre with different mean diameters. With all the minerals tested some short fibres, but not long fibres, were transported to the hilar lymph nodes. In some instances the numbers of short fibres found in these nodes appeared to be much higher than would be expected from the percentage of short fibres in the original sample, and it is suggested that this may be due to the breakdown of long fibres within the lung.