1. The specific organism should be shown to be present in all cases of animals suffering from a specific disease but shold not be found in healthy animals. 2. The specific microorganism should be isolated from the diseased animal and grown in pure culture on artificial laboratory media. 3. This freshly isolated microorganism, when inoculated into a healthy laboratory animal, should cause the same disease seen in the original animal. 4. The microorganism should be reisolated in pure culture from the experimental infection.