Mineral oils recovered from the peritoneal cavity of injected mice showed in all sixty-one instances, with the exception of liquid paraffin, a fall in the refractive index. This phenomenon is presumably due either to a chemical change (? oxidation or reduction) in the constituents of the oils brought about by the juices of the animal organism, or to a mechanical change resulting from the removal of selected constituents to other parts of the animal. This test will possibly prove to be superior to that of skin applications and estimation of the refractivity constants for measuring the amount of carcinogenic constituents in mineral lubricating oils and it has the advantage of necessitating the use of only one animal instead of 100. We are of opinion that our experiments may be of far-reaching importance. They carry us a definite step nearer to the understanding of the mechanism of cancer formation.