M cells within the follicle-associated epithelium (FAE) of the gut play a central role in the initiation of mucosal immune responses by transporting antigens to the intestinal lymphoid tissue. We have previously demonstrated that the instillation into the gut of a nonenteric microorganism, Streptococcus pneumoniae R36a, is an excellent experimental model to investigate the highly dynamic nature of the FAE in response to microbial challenge. In the present study, S. pneumoniae was introduced into rabbit ileal loops, each one containing a Peyer's patch (PP), and the number of M cells was assessed by morphological and functional characteristics in different areas of the FAE after a short time (1-3 hours). We report that a marked increase in the number of M cells was detected in the periphery, but not in the apical area, of the FAE as early as 1 hour after exposure to S. pneumoniae. Furthermore, a variant of this experiment enabled us to establish that the increased numbers of M cells led to an improved capability of the FAE to transport latex fluorescent microspheres (0.5 microm), highly specific to rabbit M cells, from the gut lumen to the intestinal lymphatic system. In these animals the cisterna chyli was cannulated, and the microparticles were introduced into the intestinal loops after stimulation with pneumococci. The microparticles reaching the lymph were then counted by flow cytometer. We interpreted these results as showing that only enterocytes located within the periphery of the FAE are converted to fully operational M cells by certain microbial interaction and the ability of enterocytes to undergo this conversion may depend on their stage of differentiation.