The efficacy of an inactivated vaccine against paratuberculosis, GUDAIR(r), was evaluated in a naturally infected herd of cattle. All the animals from the herd were vaccinated and a representative group of 100 animals was used for the study. Serum and blood samples were taken on the day of vaccination and at 21, 60, 120, 180, 240 and 300 to evaluate the humoral and cellular immune response. The intradermal tuberculin test was performed at day 0, 60, 180, 240 and 300 post vaccination. Adverse effects caused by the vaccine were not observed except for persistent nodules at the vaccination site. The vaccine produces a strong and lasting cellular and humoral immune response. Vaccinated animals reacted to both bovine and avian PPD tuberculins, but the avian reaction was usually stronger and persisted longer than the mammalian one. The vaccine highly reduced the losses due to the disease. Three months after vaccination no clinical signs were observed and a high increase in milk production was recorded. During the last four years all new replacement animals have been vaccinated and no animals with clinical paratuberculosis have been notified.