M cells present in the follicle-associated epithelium (FAE) of mouse Peyer's patches take up and transport enteric antigens to the underlying gut-associated lymphoid tissue (GALT) for subsequent processing by lymphocytes and macrophages. Present experiments use a new type of cell-labeling technique in attempts to measure the number of M cells present in the FAE of specific pathogen-free (SPF) and normal animal house reared (AH) BALB/c mice. Results show the apparent M-cell area to increase threefold 7 days after transfer of SPF mice to an AH environment. This change takes place without significantly affecting the area of FAE occupied by goblet cells. It is suggested from these results that M-cell production can be selectively increased within the FAE through the presence of foreign antigens and that this effect could have general importance in controlling the overall GALT response to enteric infection.