In a study to determine the most suitable method of expressing activity of jejunal mucosal enzymes, the effect of variation of thickness and protein concentration of biopsies was assessed. Despite wide variation of both these factors, we found no advantage in expressing enzyme activity by reference to either surface area or protein content. Expression of activity by reference to weight of biopsy gave the lowest variance for each of the enzymes studied and, since it is also the easiest measurement to make, it would appear to be the preferred means of expressing jejunal mucosal enzyme activity.