[en] The phagocytic activity of rat alveolar macrophages was studied in vitro for up to six weeks after whole-body gamma irradiation (7.5 Gy). Ingestion of fluorescent latex microspheres was evaluated using microscopic examination and flow cytometric quantitation. After a transient decrease on first day post-irradiation, a slight increase in the phagocytic ability was observed from days 14 to 28 (maximum on day 21). From day 35 onwards, phagocytic capacities returned approximately to those of the controls. The augmentation of phagocytic capacities after irradiation was characterized by an increase in the percentage of cells ingesting more than five microspheres. Flow cytometric studies showed a significant increase in mean cell size on day 21 post-irradiation. Augmentation of the phagocytic activity was not found when macrophages from control rats were incubated with serum or alveolar lavage fluid from an irradiated donor. Augmentation of phagocytic capacity was observed after 8.5 Gy but no effect was observed after 6.5 Gy. (U.K.)