The chemical potential of a compound often increases when a neutral polymer is added to the solution. This has been interpreted as an exclusion of the compound from part of the solvent. The present paper describes how the exclusion influences some enzyme reactions. A theoretical prediction based on the Michaelis‐Menten treatment is presented. Three enzyme systems have been examined experimentally, i.e.: (a) the degradation of hyaluronic acid by hyaluronate lyase in the presence of polyethylene glycol; (b) the lactate dehydrogenase reaction in dextran solution; (c) the cleavage of benzoyl dl‐arginine p‐nitroanilide by trypsin and its inhibition by serum albumin in dextran solution. In all cases, the presence of the polymer produced a decrease of the apparent Km value for the substrates and of the apparent Km value for the substrates and of the apparent Ki value for the inhibitor. The magnitude of this decrease correspond with that calculated theoretically. The effects were moderate in dextran solutions as concentrated as 0.20–0.25 g/ml.
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