This test was conducted to detect soy bean milk in cow`s milk. To detect soy milk, it`s protein was compared with cow`s milk protein by polyacrylamide gel electrophoresis (PH 8.6, Tris buffer). Differences of those proteins are summarized as follows. 1. Cow`s milk appears to be 6 protein bands, and soy milk 9 bands. 2. Electrophoretic mobility of globulin in soy milk is higher than k-casein, but lower than γ-casein, it`s quantity is very much. In case of admixture milk (soy milk in cow`s milk), 1. We can see a protein band which has higher mobility than k-casein, but lower than γ-casein in 2% (w/v) of soy milk in cow`s milk. 2. We can see another mobility protein band in admixture milk.