During the development of a sheep infection model for Johnes disease (Jd) the following parameters were studied: The age of lambs at challenge: animals challenged between 4 and 14 weeks appeared equally susceptible to infection. Dose of bacteria required to establish an infection: doses ranging from 10⁶ to 10¹² cfu were tested. The optimal range involved challenge doses between 10⁸ and 10¹⁰ cfu. Infection could be established with as few as two challenge doses. The strain of microorganisms: different laboratory isolates produced variable results in establishing infection. Microorganisms directly isolated from tissue were superior to laboratory passaged cultures. Outgrowth by fast growing avirulent bacteria may explain the reduced efficacy of laboratory cultured microorganisms. Route of infection: oral inoculation or direct instillation into the tonsilar crypt, was suitable to infect animals. The development of a robust standard infection model is essential both for vaccine efficacy studies and a better understanding of the critical pathways of immune reactivity in animals.