The lung retention of man-made mineral fibres by rats, sacrificed 5 days after the end of a 6 day period of exposure to the airborne fibre, has been examined as a function of fibre length, diameter and aerodynamic equivalent diameter. The retention of fibres < 0.5 μm dia. showed a peak of about 7.6% at 21 μm fibre length and was significantly higher than the retention (< 1%) of fibres in the diameter categories 0.5–0.7 pm and 0.7–1.0 μm. In terms of aerodynamic equivalent diameter, retention was a maximum, ca. 7%,at de = 1 μm, but decreased rapidly to about 0.5% at de between 1.5 and 2.5 μm amd 0.01% at ca. 4 μm. When fibre length only was considered, irrespective of diameter, retention fell sharply as length increased from 2 to 10 μm, then more slowly to < 0.05% at ca. 40 μm.