A simple and rapid phosphatase test is described which is based on the use ofp–nitrophenylphosphate as the substrate. Execution of the test requires few and easy operations: incubation of the milk at 37° C. with a buffered solution of the substrate, followed directly by a visual comparison of the intensity of yellow colour of liberatedp–nitrophenol with that of boiled-milk standards containing known amounts ofp–nitrophenol. The test permits the detection of minor processing faults with an incubation time of only 30 min. In this form it should prove of value in creamery control. A more searching, but equally simple test is obtained by increasing the period of incubation to 2 hr.