A new model has been developed to describe the clearance and retention of kaolin refractory ceramic fibers (RCF) in the rat lung. Fiber dissolution, breakage and differential clearance were considered in this model and their respective rates were determined from the available experimental data. It was shown that the new model offered a better interpretation of the observed fiber size distribution in the lung than our previous model (Yu et al. (1994a) Environ. Res. 65, 243-253) in which none of these effects were considered.