SummaryAn immunoassay for rat intestinal calcium-binding protein (CaBP) employing antibody raised in chickens using purified rat intestinal CaBP has been used to measure immunoreactive CaBP in the duodenum of suckling and weanling rats. A 5-fold increase in immunoreactivity, associated with a 2 1/2-fold increase in calcium-binding activity occurred at weaning, a period during which duodenal mucosal mass increased 2-fold and supernatant protein concentration increased 1 1/2-fold.