1. Using a new method of quantitative cytochemistry to determine alkaline phosphatase activity, it was possible to identify M cells and distinguish two further types of enterocytes present in mouse Peyer's patch follicle-associated epithelial tissue. 2. A quantitative description of these three cell types was provided by making direct absorbance measurements of enzyme reaction product on the luminal surface of individual follicle-associated enterocytes. M cells having a negligible alkaline phosphatase activity had a mean absorbance of 6 +/- 2 arbitrary units. The other two populations of enterocytes had overlapping but significantly different mean absorbances of 20 +/- 2 and 30 +/- 3 arbitrary units. 3. M cells accounted for 3.2 +/- 1% of the total number of cells present in the apical part of the follicle-associated epithelium. This percentage is a minimum value describing an extreme example of cells having M cell-like characteristics. The remaining epithelial cells were divided equally between those having high and low alkaline phosphatase activities in their brush-border membranes. A small percentage of cells having low alkaline phosphatase activities could perform M cell functions. 4. Results obtained suggest that reduced alkaline phosphatase activity and brush-border membrane Cl- conductance can both be used as differentiation markers to identify a large population of poorly differentiated enterocytes present in mouse follicle-associated epithelial tissue.