A technic for the early diagnosis of rabies by injecting emulsion of the brain of rabid animals into the brain of 4-6 weeks old mice is described. A special strain of mice was employed and found more satisfactory and cheaper than rabbits and guinea pigs. Diagnosis is possible earlier, and is also more accurate. Comparative results from brain material of 1,032 animals diagnosed by the usual methods were found to be positive by the brain-emulsion injection method, but in addition an error of 12% was shown for brains reported as negative for Negri bodies. Vaccination or treatment of animals with rabies vaccine apparently increased the difficulty of diagnosis by discovery of Negri bodies.