The effect of three different field isolates of Mycoplasma bovis on selected immunological parameters in experimentally infected calves was studied. Calves were kept separately in 4 experimental groups, and animals of 3 groups were infected intratracheally with one of the three selected isolates of M. bovis. The control group was inoculated intra-tracheally with sterile physiological saline. Nasal swabs and blood samples were collected just before the calf inoculation, then daily for seven days, and then weekly for another three weeks. The presence of M bovis antigen, M. bovis antibodies, total protein, gamma globulins, IgA, IgM, IgG, acute phase proteins (haptoglobulin and serum amyloid A), as well as interferon-γ and interleukin-4 concentrations were determined. M. bovis was detected intermittently during the study in the infected groups from day 1, whilst the control group remained free of the pathogen. M. bovis antibodies were detected in some of infected animals in the second, third, and fourth week arter infection. The stimulation and/or immunological suppression varied between the M bovis isolates used for the inoculation. All M. bovis isolates induced a rise of APP and gamma globulin concentrations in infected calves. However, in this study the mucosal immune response appeared to be down-regulated, which was expressed with a general lack of IgA stimulation.