SUMMARY1. Prochymosin-and pepsinogen-containing cells in abomasal mucosa were simultaneously localized by means of an indirect immunofluorescence technique with specific rabbit antibodies.2. In the young milk-fed calves all the chief cells and several mucous neck cells produced both prochymosin and pepsinogen.3. By elution of the first antibody and retaining of the same section it was demonstrated that the same cells were producing both prochymosin and pepsinogen.4. In concentrate-fed calves and older cattle all chief cells and almost all mucous neck cells produced pepsinogen, while prochymosin was produced only from the chief cells situated in the upper part of the base in the gastric gland. 5. The number of prochymosin-containing cells was highly correlated to the milk-feeding of the animal.6. The present data suggest that the non-differentiated cell in the neck region is initially capable of producing both prochymosin and pepsinogen during the proliferation phase to a mature chief cell, via mucous neck cell. The ability to produce prochymosin is however only retained if the mucosa is stimulated by milk-feeding. Dietary factors may thus be of importance for the differentiation of the gastrointestinal epithelium.