Prevalence of paratuberculosis has been monitored in a herd with 1700 cows for a long period. All animals have been examined serologically by AGID and CFT since 1990 and also by culture of faecal samples twice a year since 1993. M. avium subs. paratuberculosis isolates were identified by subculture and PCR (probe IS 900). Positive by faecal culture at the beginning of regular examination was 5.6% animals. There were relatively a lot of animals (23.1% from slaughtered cows) with only one positive finding in a faeces during the period of investigation and with negative results of pathological and microbiological examinations of intestines and mesenteric lymph nodes after slaughter. This animals, although infected, was considered resistant. Animals shedding repeatedly M.a. paratuberculosis in faeces (22.2% were low shedders, 28.2% were high shedders) and with findings after slaughter, were considered as susceptible. They were divided into the subgroups according their clinical status. In 1997 - 1998, the selected animals were tested immunologically. The set of techniques included flow cytometry, phagocytosis and chemiluminescence tests, lymphoblast-transformation test after stimulation with non-specific mitogens and PPD antigen of M. avium and determination of IFN-gamma after the stimulation of cells with specific antigen. The number of serologically positive animals increased with growing intensity of shedding of M. a. paratuberculosis, but animals positive by serological tests and negative by culture were also found. Positive results of cell-mediated immunity test, in particular the production of IFN-gamma, were obtained mostly in the animals classified as resistant. As to the parameters of non-specific activity of the immune system, larger differences were found between resistant and susceptible animals, than among subgroups of susceptible infected animals at various stages of the disease. Increased phagocytic activity was found in most animals affected by diarrhoea, however. The counts of lymphocytes and counts of the CD8+ cells were significantly lower in the subgroup of susceptible animals at an early clinical stage than in the subgroup of resistant animals with a minimal shedding of mycobacteria. (Supported by GACR 524/97/O948).