The use of the p-nitrophenyl-alpha-L-rhamnopyranoside for the specific measurement of the alpha-rhamnosidase activity of naringinase, by colorimetrically following the appearance of p-nitrophenolate anion, is proposed. Use of this synthetic substrate did not change the pH, temperature, or ionic strength optima of the enzyme. It did, however, result in (a) a decrease of the Michaelis constant of the enzyme, allowing the Vmax to be measured, this being impossible to accomplish with naringin, (b) an increase in the sensitivity of the assay to the presence of inhibitors in the reaction media, (c) an increase in the sensitivity which enabled measurement of low levels of naringinase due to the high absorptivity of p-nitrophenolate, and (d) a quick and cheap method of evaluating the alpha-rhamnosidase activity of naringinase.