A simple and sensitive assay is described that can detect trace amounts of whey cream in sweet cream and in butter made from that cream. The method is based on the detection of sialic acid and involves a color reaction between Ehrlich's reagent and sialic acid. The procedure is faster than other methods for detection of whey. The method could be termed semiquantitative because of the seasonal variability of sialic acid in cow's milk. However, with a calibration curve, the method is quantitative and can easily detect as little as 1% whey cream in sweet cream and in the resulting butter. With 1% whey cream in sweet cream, the purple color indicating the presence of sialic acid is visible to the naked eye.