Proteins of the basolateral membrane (BLM) of small intestine epithelial cells of adult rats, in the MW ranges of 50-65 KD, 85-100 KD, and over 100 KD, were obtained as follows. After isolation of the BLM and subsequent SDS-PAGE and transblotting of the proteins on nitrocellulose sheets, the bands in these MW ranges were cut out of the nitrocellulose sheet and extracted. Balb/C mice were immunized with these protein fractions and a monoclonal antibody (MAb) was then produced. MAb SI/CC1 obtained via immunization with the 50-65 KD protein fraction shows specificity for the crypt epithelium of the small intestine. It can be used to characterize, by light and electron microscopic immunohistochemical methods, a crypt cell protein (SI/CC1-Ag) with a very specific localization. Fluorescence labeling shows that the SI/CC1-Ag can be found only in the epithelium of small intestine crypts (except for the granules in eosinophilic granulocytes). The epithelium of the colon, as well as the epithelia of other organs, could not be labeled. In the small intestine crypts, SI/CC1-Ag is found only in the Paneth cells located in the basal crypt section, and in the undifferentiated cells in the middle crypt section; it is lacking in the cells of the upper crypt section. Gold labeling shows that SI/CC1-Ag in the undifferentiated cells is localized exclusively in the basolateral PM domain. On the Paneth cells, the content of the secretory granules is labeled, along with the basolateral PM domain; the labeling sometimes present on their luminal part is probably due to passively absorbed secretion from these cells. The SI/CC1-Ag in the BLM of undifferentiated and Paneth cells is found only on Days 21-23 post partum, whereas the Paneth cell granules could be labeled as early as the Day 16 post partum. With immunodetection with SI/CC1, one band at about 55 KD is specifically labeled in the protein pattern of the isolated small intestine cell BLM. In the protein pattern of the isolated crypt cells two bands were labeled, again one at 55 KD and one at about 120 KD. These findings indicate that SI/CC1-Ag is a 55 KD protein that appears on Days 21-23 post partum in the BLM of undifferentiated cells and of Paneth cells.