Rats were exposed to quartz dust (about 90 mg/m3) for five hours a day, five times a week either throughout the 48 weeks of the experiment or for a total of 40 weeks plus eight weeks of "rest." Cytological study of bronchoalveolar lavage showed that at a certain level of silicotic changes in the lungs, a pronounced breakdown in pulmonary dust clearance by macrophages could be observed. There was, however, a concomitant compensatory increase in the contribution to pulmonary phagocytosis by the neutrophil leukocytes (NL). As a result, pulmonary dust kinetics could be mathematically described using a constant clearance rate both throughout the 48 weeks of exposure and during the eight weeks after the end of the 40 week exposure. It is shown in a separate experiment that enhanced recruitment of NL into the airways may be due not only to an attractant effect of the products of macrophage breakdown (PMB) themselves but also to the release of an NL attractant factor by viable macrophages activated under influence of the PMB.