SummaryThe cytochemical and functional characteristics of broncho‐alveolar multinucleate giant cells and the kinetics of the giant cell response in the lungs of mice and rats duringNippostrongylus brasiliensisinfection were studied. Primary infections resulted in significantly increased numbers of recoverable giant cells for up to 30 and 50 days in rats and mice, respectively. During secondary infections in the rat the giant cell response was more rapid and greater in magnitude than in a primary infection, suggesting that it was immunologically mediated. The giant cells displayed decreased C3‐ and IgG‐dependent binding or phagocytic potential compared with mononucleate alveolar macrophages. Fusion of mononucleate alveolar macrophages into giant cells may therefore compromise complement and antibody dependent helminthocidal activity of these