An instrument for measuring the size of a testis has a pair of relatively displaceable jaws with testis contacting surfaces. The spacing between the jaws is displayed on a scale calibrated to a recommenced dose of a chemical sterilant at a given concentration for the testis measured for sterilizing a male animal having scrotal testes. The instrument is used to determine the appropriate dose to be administered the animal, each testis of which should be independently measured as the size of the testicles may different in the same animal.