M cells are specialized structures in the Peyer's patch follicle-associated epithelium capable of taking up bacteria, viruses and other pathogens for later presentation to the gut-associated lymphoid tissue. The present work studies how coating microspheres with different proteins affects their ability to be taken up by M cells under near physiological conditions in vivo. The later appearance of microspheres in intestinal lymph has also been measured by flow cytometry. The protein preparations used in these experiments included bovine serum albumin (bSA), human immunoglobulin G (hIgG), secretory immunoglobulin A (hIgA), bovine growth hormone (bGH) and bGH complexed with an IgG antibody raised against bGH (bGH-Ab). Selectivity in binding of these microspheres to M cells, determined by confocal microscopy, was bGH < bSA < hIgG (mice) and bGH < bGH-Ab (rats and mice). A similar selectivity was seen for microsphere entry into M cells (bGH < bSA < hIgG; bGH < bGH-Ab). The appearance of protein-coated microspheres in rat mesenteric lymph showed a similar selectivity to that found for binding and entry into M cells (bGH < bGH-Ab). This latter selectivity was also found for hIgA-coated microspheres (bSA < hIgA). Preservation of transport selectivity throughout transcytosis highlights the unique importance of the M cell surface as being the primary site determining which type of antigen can be presented subsequently to the gut immune system. The possibility that this is a transient or phasic property of the M cell surface and that this could have physiological relevance is also discussed.