A method has been devised whereby the potency of any given carcinogenic agent can be compared with that of a standard agent. This is called the standard carcinogenic potency of the agent (P.). The relative potency of the agent (R.P.), calculated from P., permits one to estimate the effect on the agent of different chemical treatments, dilutions, etc., the percentage increase or decrease in activity of the agent being at once shown. A unit of carcino-genicity (U.C.) has been established, and a concentration of 500 carcinogenic units per cubic centimetre is considered to induce the maximum relative response on animals when the agent is applied twice a week. These standards have been devised as a natural sequence to the accumulation of a vast amount of experimental data the correlation of which necessitated some expedient more rapid and covering a wider field than the graphs formerly utilized.