Early detection of M. paratuberculosis infection, using diagnostic tests suitable for calves, would accelerate progress in control programs for Johne's disease. Limited fecal shedding of M. paratuberculosis and seroconversion in cattle less than two years of age has been reported in previous studies. The aim of this study was to evaluate use of current diagnostic tests in detecting infected calves. Twenty-nine calves were obtained by purchase of parturient cows from properties of known Johne's disease status. At birth, the calves were allocated to four groups as follows, (1) control group of four uninfected calves; (2) vaccinated group of eight calves immunized with either the Weybridge vaccine (CVL, Weybridge) or a heat-killed field strain of M. paratuberculosis and three unvaccinated calves; (3) artificially infected group of eight calves stomach drenched with either 2g wet weight of M. paratuberculosis or 20g of macerated intestinal mucosa from a clinically infected cow; and (4) naturally infected group of five calves reared by dams that were positive for M. paratuberculosis by fecal culture. Vaccines were administered to group two calves within twenty days of birth and group three calves were challenged three times, at weekly intervals, wthin one month of birth. Blood and fecal specimens were collected monthly until the calves were slaughtered between the ages of 20 and 30 months. Results from histopathology, tissue and fecal culture, humoral and cell-mediated tests will be presented. In preliminary testing a small number of reactors to the absorbed EIA and fecal culture have been detected. Four calves from groups two, three and four tested positive for serum antibody to M. paratuberculosis using the Johne's Absorbed EIA (CSL, Australia). M. paratuberculosis was isolated by fecal culture from five calves in group three using modified Roche MB Check system (Becton Dickinson, Australia) and double incubation radiometric culture. Strategies for testing young cattle will be discussed.