The pathogenicity for calves of two strains of wood-pigeon mycobacteria isolated in our laboratory, one strain isolated in the United Kingdom, one strain isolated from a Roe-deer in Denmark and three strains of M. paratuberculosis, were compared. Seven groups of four 3 to 4 week-old calves were infected by the intravenous route using 10(6) to 10(9) viable units. They were slaughtered one year after the exposure or earlier if their state required it. The macroscopic and microscopic lesions and cultures from the mesenteric lymph nodes, ileon and caecum were studied. At necropsy, the post-mortem findings were hardly significant. However, the histopathological studies showed lesions identical to the classical aspects of paratuberculosis in cattle, both in animals inoculated with M. paratuberculosis and those inoculated with wood-pigeon mycobacteria. The isolation of the bacteria showed that all animals were infected. The experimental infection was able to reproduce a disease resembling paratuberculosis with M. paratuberculosis (except the strain 2103) and with wood-pigeon mycobacteria, but it has not been able to distinguish the disease caused by M. paratuberculosis from the one caused by wood-pigeon mycobacteria.