Suspensions of BCG (32-80 X 10(7) viable bacilli/ml) were injected into constricted loops of ileum containing a Peyer's patch from non-immunized young adult rabbits after laparotomy under anesthesia. Light microscopically (rhodamine B-auramine double staining, Ziehl-Neelsen staining), many mycobacteria were observed adhering to the surface of follicle-associated epithelium (FAE) and in phagocytes that had migrated within FAE and to deep lymphoid follicles. Electron microscopically in 1-hour post inoculated specimens, the bacteria were found adhering specifically to M cells, and the microfolds of the M cells were seen to stretch like tentacles toward the bacteria to catch them. The bacteria were recognized in the large vesicle of the cytoplasm of the M cells. The bacteria phagocytized by the M cells were found in the intercellular space between M cells and columnar cells. The bacteria were also seen in macrophages enfolded by the M cells as well as in macrophages below the FAE. In this experiment, BCG were found to have been phagocytized only by M cells, and none were found in absorptive cells. It was verified that M cells are the first entrance through which BCG invade the host and that they are transported from M cells to macrophages of the epithelium.