Rabbits and rats were exposed in a chamber to coal (20,000 and 10,000 particles/ml) and hematite (20,000 and 37,000 particles/ml) in succession to follow deposition and clearance by color. Exposures were 5 days/wk, 20 hr/day. Generally, there was dust widely but not uniformly distributed in peripheral alveoli, tending to aggregate and clump in more proximal alveoli with time. There was initial nonuniform distribution more uniform with exposure time. Aggregation mostly through phagocyte activities and more evident in rabbits than rats was observed. There was eventual mixing of two dusts inhaled up to 7 months apart. Dust deposited last is cleared first because of less tortuous route.