An experimental oral infection of goats with a caprine isolate of Mycobacterium avium ss paratuberculosis was used to investigate immunological and bacteriological changes during the first year post-infection. Seven 2-3 months old goats were given a bacteria-suspension in milk three times weekly for nine weeks. Six animals were kept as controls. The lymphocyte response following in vitro stimulation with specific antigens was characterised using the lymphocyte proliferation test, the gamma-IFN immunoassay and the IL-2 receptor test (flow cytometry). The antibody response was assayed using an ELISA technique. Lymphocytes from infected animals responded significantly within nine weeks in all three tests, and still do one year after infection. Four infected animals showed a weak serological response as early as 15 weeks post infection and all were serological positive after one year. The bacterial examination of the faeces was negative during the first year post infection.