Uptake of particulate antigenic matter, including microorganisms and vaccine-bearing microspheres, by the intestinal mucosa takes place in the domes of the gut-associated lymphoid tissues and is achieved by membranous (M) cells, which continuously transport particles from the lumen to the underlying tissue where some particle components initiate immune reactions. Using yeast as tracer, we investigated the kinetics of particle uptake in the Peyer's patches of pigs. A suspension of baker's yeast (Saccharomyces cerevisiae) was injected into the gut lumen of anesthetized minipigs; the position of yeast cells in the tissue was determined after 1, 2.5, 4, and 24 h using fluorescence light- and thin-section electron microscopy. After 1 h, 18.5% of all M cells had taken up or were in close contact with yeast cells. The intercellular space of the epithelium contained a maximum of 60.8% of all yeast cells found in the tissue after 2.5 h, but only 1.3% had been phagocytosed by macrophages. After 4 h most yeast cells (77.8%) were found beneath the basal lamina, and most of these (89%) were found in macrophages. No yeast cells were detected in the Peyer's patch domes 24 h after application. The data show that transcytosis of yeast particles (3.4 ± 0.8 µm in diameter) by M cells takes <1 h. Without significant phagocytosis by intraepithelial macrophages, the particles migrate down to and across the basal lamina within 2.5-4 h, where they quickly get phagocytosed and transported out of the Peyer's patch domes.