The purpose of the present study was to investigate the morphological changes in the surface of rat alveolar macrophages (AM) after their exposure to man-made vitreous fibers (MMVF). Also the ability of rat AM to phagocytize respirable-sized MMVF was studied. The study was carried out by exposing rat AMs in a temporal fashion, for from 30 min to 96 hr, to the fibers in vitro. Scanning electron microscopy (SEM) was used to demonstrate the morphological changes and the phagocytosis of MMVF by rat AMs. Before exposure, the cells had continuous membranes with a variety of small surface features. The phagocytosis of MMVF by AMs started within 30 min after the exposure and increased as a function of time. Also the formation of prominent rufflings and blebs increased by the time. Short fibers, less than 20 microns in length, were usually phagocytized by a single AM, whereas two or more AMs usually phagocytized longer fibers. The cells produced extensions which fasten them to the fibers or to other cells to form clumps or clusters of cells and fibers, each AM engulfing a part of a fiber. Over 70% of the exposed cells were viable and still active after 96 hr of exposure. This finding suggests that the MMVF were not acutely toxic to the rat AMs in vitro. The results also reveal that the surface morphology of the AMs changed slowly during exposure of MMVF and that the cells actively phagocytized MMVF even 96 hr after the beginning of the exposure.