Summary1. A method is described for investigating the possible action of microdoses of mercuric chloride on the hydrolysis of soluble starch with malt diastase.2. The microdoses of the mercuric chloride used in the latest crucial series carried out in1946, 1948, and 1952, were what are termed “high potencies” made in accordance with the pharmaceutical method of preparation of drugs ordinarily used in the practice of homœotherapy.3. These microdoses were prepared by separate stages of dilution, the solution at each stage being subjected to mechanical shock. The solutions were, theoretically, “dilutions” of the order of 1 in 10−61 and on present physical theory would not contain any molecules of the original mercuric chloride.4. The difference in rate of hydrolysis between flasks containing starch, diastase, and distilledwater (controls) and flasks containing starch, diastase and microdoses of mercuric chloride (tests) were compared colorimetrically by the Spekker absorptiometer, and the frequencies of the differences statistically analysed, as the results obtained showed biological scatter. More than 500 such comparisons were carried out. The differences of means were examined by the Fisher “t” test, the variances tested and Cochrane and Cox's test applied where indicated. All the series gave a highly significant difference in the rate of hydrolysis between controls and tests, the microdoses stimulating the process. Statistically the significance is shown by the fact that a probability of <0·001 was obtained independently in each of the three years 1946, 1948 and 1952. The control results gave an approximately normal distribution.5. The distribution, control methods, and accessory control procedures were considered toexclude, as a cause of the effects, adsorption of the original drug and the presence of extraneous contaminants by chance solely in test flasks. The only difference between control and microdose flasks was the addition of microdose, the distilled water being common to both controls and tests.6. It was concluded that a factor, unidentified, derived from the mercuric chloride used, waspresent in solutions prepared by serial dilution with mechanical shock which could affect the distilled water diluent, that this change was transferable to subsequent “ultramolecular” stages of “dilution”, and that this factor was the source of the activity in the microdose solutions producing the acceleration of the rate of hydrolysis.7. In an addendum there is described recent biological work which is also providing evidence of the presence of an active selective factor in “high potencies” derived from Strophanthus sarmentosus by the same methods of dilution with mechanical shock.