A crystalline trypsin inhibitor from soybean has been isolated and characterized. The inhibitor is a globulin-type protein with a molecular weight of about 24,000 and an isoelectric point at pH 4.5. It is stable in a wide pH range and is not affected by heat, acid, or alkali unless denatured, which reduces its inhibitory activity. The inhibitor forms an irreversible compound with trypsin, inhibiting its proteolytic activity. It slightly inhibits chymotrypsin, but the inhibition is reversible and follows the law of mass action. The inhibitor has no effect on pepsin's proteolytic activity or milk-clotting ability. The inhibitor is stable and can be used as a standard for measuring trypsin activity. The protein is precipitated by trichloroacetic acid and is non-diffusable through membranes. It contains less than 0.01% phosphorus and is free of carbohydrates. The inhibitor's activity is lost upon denaturation, but it can be reversed. The inhibitor is readily digestible by pepsin, chymotrypsin, and trypsin when denatured. The inhibitor's activity is measured by comparing the proteolytic activity of trypsin samples with and without inhibitor. The specific activity of the inhibitor is about 1.0 when expressed in terms of weight of pure trypsin inhibited. The inhibitor's properties are determined using spectrophotometric methods, including measuring light absorption at 280 mμ. The inhibitor's stability and purity make it a suitable standard for assaying trypsin samples. The inhibitor's inhibitory action is associated with its native state, and denaturation reduces its activity. The inhibitor's interaction with trypsin is irreversible, while its interaction with chymotrypsin is reversible. The inhibitor's activity is measured using a standard curve based on optical density readings. The inhibitor's properties are summarized in a table, including its chemical and physical characteristics. The study provides a detailed analysis of the inhibitor's properties, including its stability, activity, and interactions with various enzymes. The inhibitor's activity is measured using methods such as gelatin-formol titration and casein digestion. The inhibitor's activity is independent of the method used to measure trypsin's proteolytic activity. The inhibitor's activity is also measured using a colorimetric method with a Cu-phenol reagent. The inhibitor's activity is expressed in terms of trypsin inhibited and is calculated based on the difference in proteolytic activity between samples with and without inhibitor. The inhibitor's activity is also measured using a spectrophotometric method, with the optical density at 280 mμ used to determine protein concentration. The inhibitor's activity is independent of the purity of the trypsin sample used. The inhibitor's activity is measured using a standardA crystalline trypsin inhibitor from soybean has been isolated and characterized. The inhibitor is a globulin-type protein with a molecular weight of about 24,000 and an isoelectric point at pH 4.5. It is stable in a wide pH range and is not affected by heat, acid, or alkali unless denatured, which reduces its inhibitory activity. The inhibitor forms an irreversible compound with trypsin, inhibiting its proteolytic activity. It slightly inhibits chymotrypsin, but the inhibition is reversible and follows the law of mass action. The inhibitor has no effect on pepsin's proteolytic activity or milk-clotting ability. The inhibitor is stable and can be used as a standard for measuring trypsin activity. The protein is precipitated by trichloroacetic acid and is non-diffusable through membranes. It contains less than 0.01% phosphorus and is free of carbohydrates. The inhibitor's activity is lost upon denaturation, but it can be reversed. The inhibitor is readily digestible by pepsin, chymotrypsin, and trypsin when denatured. The inhibitor's activity is measured by comparing the proteolytic activity of trypsin samples with and without inhibitor. The specific activity of the inhibitor is about 1.0 when expressed in terms of weight of pure trypsin inhibited. The inhibitor's properties are determined using spectrophotometric methods, including measuring light absorption at 280 mμ. The inhibitor's stability and purity make it a suitable standard for assaying trypsin samples. The inhibitor's inhibitory action is associated with its native state, and denaturation reduces its activity. The inhibitor's interaction with trypsin is irreversible, while its interaction with chymotrypsin is reversible. The inhibitor's activity is measured using a standard curve based on optical density readings. The inhibitor's properties are summarized in a table, including its chemical and physical characteristics. The study provides a detailed analysis of the inhibitor's properties, including its stability, activity, and interactions with various enzymes. The inhibitor's activity is measured using methods such as gelatin-formol titration and casein digestion. The inhibitor's activity is independent of the method used to measure trypsin's proteolytic activity. The inhibitor's activity is also measured using a colorimetric method with a Cu-phenol reagent. The inhibitor's activity is expressed in terms of trypsin inhibited and is calculated based on the difference in proteolytic activity between samples with and without inhibitor. The inhibitor's activity is also measured using a spectrophotometric method, with the optical density at 280 mμ used to determine protein concentration. The inhibitor's activity is independent of the purity of the trypsin sample used. The inhibitor's activity is measured using a standard