Results from studies of the retention of contrasting mineral dusts inhaled by rats (for periods of up to three months) and the resultant changes in the phagocyte defence system of the deep lung were examined. The dusts used were titanium dioxide (relatively innocuous) and quartz (relatively toxic). The parameters assessed included the accumulation of material in the lung and lymph nodes during chronic exposure and the associated leukocyte response as assessed by broncho-alveolar lavage. The principal findings were that: (a) low level exposure to titanium dioxide produced no measurable inflammation (as indicated by neutrophil recruitment) but higher concentrations (30, 50, and 90 mg/m3) caused the transfer of dust to lymph nodes and first evidence of inflammation; and (b) for quartz, there was a more prominent response and earlier transfer of material to the lymph nodes. The suggested relation between changes in the neutrophil population and dust accumulation is discussed in terms of a quantitative dosimetric model, from which implications for assessing and managing human exposures emerge.