Rats were exposed to three concentrations of diluted diesel exhaust for 6 mo and 1 yr. Bronchopulmonary lavage was used to obtain the pulmonary phagocytes from the animals in order to study the response of the phagocytic defenses to the inhaled particulate. The cell numbers and volumes were measured using an electronic particle counter. The cell counts of alveolar macrophages (AM) were proportional to the concentration of diesel exhaust particulate (DP) in the chronic exposures. AM increased in the lungs in response to the rate of DP mass entering the lungs, rather than to the total DP burden in the lung. The geometric mean volumes of AM from the exposed and control animals were approximately 1100 micron3 at both 6 and 12 mo of exposure, although exposed cell-volume distributions skewed towards larger sizes. The AM volume distributions extended to 2000 micron3 in both control and 250 micrograms DP/m3 exposed animals and up to 5000 micron3 in cells from animals exposed to 750 and 1500 micrograms DP/m3. The volume distributions were found to be reproducible in equivalent control and exposed cell populations. Polymorphonuclear leukocytes were present in the lavaged cell populations from the animals exposed to 750 and 1500 micrograms/m3. In addition, at 1 yr of exposure, lymphocytes were also lavaged from animals exposed to 750 and 1500 micrograms DP/m3. Protein, beta-glucuronidase activity, and acid phosphatase activity were measured in the lavaged cells, and were elevated in the cells from animals exposed to 750 and 1500 micrograms/m3. The buoyant density of diesel-laden AM was found to be greater than that of control AM, and overlapped with the buoyant density of the polymorphonuclear leukocytes.