A major field trial was undertaken from 1999 until 2004 to determine the efficacy of a killed M. a. paratuberculosis vaccine, Gudair™, for the control of ovine Johne's disease (OJD) in Australian merinos run under Australian pastoral conditions. On each of three farms in New South Wales experiencing significant OJD losses (5 to 15% per annum), 200 Merino lambs (age 1-4 months) were vaccinated with Gudair™, and 200 lambs were sham vaccinated with saline. Animal assessments and sample collections were conducted twice yearly until 4 or 5 years of age. We examined the impact of vaccination on mortality rate, faecal shedding of M. a. paratuberculosis (by pooled and individual faecal culture), lamb growth, condition score and wool productivity, vaccine injection site lesions and cellular (BOVIGAM™) and humoral (PARACHEK™) immunity. Gudair™ reduced mortalities due to OJD by about 90% and delayed faecal shedding of M. a. paratuberculosis for the first year post-vaccination (pv). Thereafter, the prevalence of shedders among vaccinates was reduced by about 90%. The total numbers of M. a. paratuberculosis excreted by the vaccinated groups were also reduced by at least 90% (one to several logs lower) at most sampling times. However, high levels of excretion by vaccinates were found on some occasions, and although overall only 7 of 600 vaccinates died of OJD, all 7 had multibacillary disease. Thus there remains a risk that some vaccinated sheep could transfer the disease. Small (less than 200g per animal) reductions in liveweight gain were found in vaccinated lambs in the first year pv, but no significant differences were found in condition score and wool productivity. Vaccine injection site lesions were detected in almost 50% of sheep 2 months pv, and these persisted for at least 4 years in 20-25% of vaccinates. The vaccine stimulated both cell-mediated and humoral immune responses in a high proportion of vaccinated lambs which declined over time, accompanied by a significant increase in the proportion of unvaccinated animals with positive immune reactions, presumably reflecting an increasing prevalence of OJD in this group. Data from this trial enabled the registration of Gudair™ in Australia in 2002 and the vaccine now plays a pivotal role in the management of OJD in Australia.