The Kunitz method and the casein precipitating test (CP-test) have been compared for detection of proteolytic enzyme activity. For all the enzymes tested, except for ficin, the sensitivity of the CP-test was 1200 1300 x greater than for the Kunitz method. For ficin the sensitivity was approximately 250 x greater. There was a close correlation between the casein precipitating inhibition test (CPI-test) and the Kunitz method when testing the effect of inhibitors towards different enzymes. Of the systems examined, only soybean trypsin inhibitor had to be used in a high concentration in order for the inhibitory effect of this inhibitor towards α-chymotrypsin to be demonstrated by the CPI-test. With the other enzyme-inhibitor systems a great variation in enzyme and inhibitor concentrations could be used for the successful demonstration of inhibitory effect. Some of the inhibitors included showed a low inhibitory effect upon enzymes tested by the CPI-test which could not be demonstrated by the Kunitz method. This is assumed to be due to the higher sensitivity of the CPI-test as compared to the Kunitz method. The CPI-test is considered to be of value especially when working on inhibitors and enzymes of low activities and when the screening of many different inhibitors against various enzymes is to be carried out. Also in purification work the CPI-test is found to be effective.