Abstract Twelve medicine agents and chemicals were assessed for microdose effects on the enzymes urease, diastase and trypsin. Silver nitrate and mercuric chloride had a slight effect on urease; mercuric chloride, sulphur and after them calcium chloride had marked effect on diastase, microdoses of Iris, phosphoric acid and arsenic on trypsin. Gold chloride had only a slight effect on diastase. The remaining preparations—benzoic acid, platinum chloride and insulin—had no effect. An investigation was made to determine the effect of dynamized dilutions of iodine on starch, and the pH of phosphoric acid dilutions by electrometric methods. It has been empirically shown that colorimetry provides the most sensitive method for assessing microdose effects (dilutions up to 10 −100).