It is known that dietary carbohydrates regulate the activity of the intestinal SGLT1. We have demonstrated that modifications in SGLT1 activity are due to alterations in SGLT1 expression in response to the sugar content of the diet. To assess the correlation between changes in the activity of SGLT1 and the abundance of SGLT1 protein, we have employed a method for the quantitative measurement of immunoreactive proteins. A calibration curve has been constructed using either a nonadecapeptide (amino acids 402-420), or a recombinant protein corresponding to amino acids 554-640 of the SGLT1 sequence. Immunoblotting the protein samples concurrently with specific quantities of either the peptide or recombinant standard, using antibodies raised against these antigens, enabled accurate quantification of the absolute amounts of immunoreactive protein in the samples. The amount of SGLT1 protein correlates well with measurements of SGLT1 activity. The modulation of the activity of SGLT1 in response to lumenal sugars is due to corresponding changes in the absolute levels of SGLT1 protein.