SummaryThe authors review the effect produced by paired action of gaseous pollutants and microorganisms on alveolar macrophages. When tobacco smoke acts in vivo, the number of alveolar macrophages harvested from lung washings, in man or animals, is most often increased. The treated phagocytes show the characteristics of«activated»macrophages: marked glass-adherence and cytoplasm filled with various organites (lysosomes, Golgi vesicles, mitochondriae and residual bodies). Although the phagocytic properties of the cells are preserved, a reduction of their germicidal activity towardsEscherichia coliandStaphylococcus aureus,is observed in the guinea-pig. When the macrophages cultivated in vitro, are exposed to tobacco smoke the pollutant depresses their physiologic activity: decreased glass-adherence, pycnotic aspect of the nucleus, vacuolisation of the cytoplasm, reduction of the phagocytic and germicidal activities towardsStaphylococcus albusandepidermidis.Enhancement of infectious processes by nitrogen dioxide has been demonstrated in monkeys, hamsters and mice, when the animals were first exposed to the gas and then infected with an aerosol prepared withKlebsiella pneumoniaeor withStaphylococcus aureus: a reduction of the pulmonary clearance of the germs was observed. Experiments carried out on guineapig alveolar macrophages cultures exposed to nitrogen dioxide, reveal a reduction of the phagocytic activity of these cells towardsStaphylococcus aureus.Sulfur dioxide exerts a cytotoxic effect on rat alveolar macrophage? cultivated in vitro in contact with the gas, as well as a decrease of cellular viability. The latter observation contrasts with a stimulatory effect on carbohydrate metabolism and an increase of several enzymatic activities. Ozone exerts a cytotoxic action, as demonstrated in vivo on rabbit alveolar macrophages, as well as in vitro on rat alveolar macrophages. Moreover, an inhibition of microbial phagocytosis in the alveolar macrophages was observed in rabbits infected withStreptococcusgroup C Lancefield by tracheal route. On the whole, the gaseous polluants studied here exerted an inhibitory action on the anti-infectious function of the alveolar macrophage, which is more pronounced in vitro than in vivo; the inhibitory action affects the germicidal rather than the phagocytic function of these cells.