Untersuchungen über die Bedeutung der Mikroorganismen für die Entstehung der Diphtherie beim Menschen, bei der Taube und beim Kalbe.

Loeffler succeeded in cultivating C. diphtheriae, the diphtheria bacillus. He reproduced the characteristic membrane by swabbing the mucous membranes of various animals with pure cultures of the bacillus.
In this paper Loeffler, who was working as one of Koch's assistants, stated on p. 424 three postulates similar to Koch's 4 postulates:
1. The organisms must be shown to be constantly present in characteristic form and arrangement in the diseased tissue.
2. The organisms which, from their behavior appear to be responsible for the disease, must be isolated and cultivated in purity.
3. The pure cultures must be shown to induce disease experimentally.

D. Jay Grimes, "Koch's postulates- then and now," Microbe 1 (2006) 223-228.