Johne's disease (paratuberculosis) is a chronic, granulomatous infection of the intestinal tract of ruminants caused by Mycobacterium avium subspecies paratuberculosis (MAP). There is no approved treatment, no known way to eliminate the infection once established, nor is there an effective vaccine for the disease. Johne's disease (JD) has emerged as an important disease of cattle due to the economic impact and the potential link to human Crohn's disease. The recently introduced herd Risk Assessment (RA) and Herd Management Plan (HMP) developed by the National Johne's Working Group (NJWG) have been incorporated into the national program standards by USDA, APHIS. Increased state and federal funding for Johne's disease has increased the focus on ways to control the condition. Methods to control the spread and to reduce within herd transmission of Johne's disease (JD) are being adopted by both dairy and beef herds in many states through implementation of the national Johne's disease program. Implementation of best management practices (BMP) following a herd risk assessment (RA) designed to reduce the risk of transmission of JD remains the focal point of the national effort to reduce the prevalence of JD in cattle herds today. Although a vaccine for JD is available in many states, its use is closely regulated by individual state veterinarians and usage by the profession remains less than 2,000 herds nationally. Test and culling of culture-positive cattle seems a low priority if the infected animals are profitable. Reduction of the herd bioburden of MAP seems a lower priority for many producers. Brumbaugh et al1 demonstrated a reduction in the number of colony forming units (cfu) of MAP from the livers of experimentally MAP-infected mice treated with monensin compared to non-treated controls. More recently he showed monensin reduced the severity of histological lesions in cattle with clinical signs of Johne's disease, including weight loss and diarrhea.2 Initial infection with MAP is generally considered to occur in the neonatal calves. Our laboratory at the University of Pennsylvania, New Bolton Center has successfully induced experimental MAP infection in neonatal calves via oral gavage of MAP on two consecutive days.4 The current experiment was designed to assess the efficacy of monensina to reduce pass-through fecal shedding and to reduce tissue bacterial load (bioburden) of MAP in calves.