Alveolar macrophage (AMO) were recovered by bronchoalveolar lavage from mice. We have examined the surface of these cells for receptors of immune effector molecules and compared the normal resident AMO to the 'activated' cell present during extensive lung inflammation caused by the parasite Nippostrongylus brasiliensis 2 days after subcutaneous inoculation. As has been shown previously, the resident alveolar macrophage expresses Fc receptors for IgG (90%) and IgE (17%). We now show that a significant proportion of resident AMO possess an Fc receptor for IgA (14%) and this proportion increases to nearly 30% upon activation of the cell, coincident with an increase in release of plasminogen activator and phagocytic activity. This newly described presence of IgA receptors on AMO represents an important demonstration of potential 'arming' of these cells by the predominant antibody in mucous secretions and direct involvement of the AMO in immune mediated reactions in the lung.