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The intratracheal instillation of moderate doses of PbO to Long-Evans strain rats resulted in a significant increase in the number of recoverable pulmonary alveolar macrophages. The viability of recovered cells was remarkably constant throughout the experimental period of 40 days. The results obtained indicate a low order of toxicity of PbO to alveolar macrophages and show that the mucociliary escalator is a significant route of excretion of PbO from the respiratory tract. The in vitro survival of macrophages from PbO-treated rats was significantly reduced. Survival of cells from both treated and control animals was somewhat enhanced by the addition of formalinized lymphocytes to the culture medium. Morphological changes and evidence of phagocytosis are discussed.