TMA (thermostable macromolecular antigens) are major mycobacterial complexes present in all mycobacteria. We have purified A36, the TMA complex of M. paratuberculosis, the etiological agent of paratuberculosis (Johne's disease), and shown by the immune electron microscopy approach its presentation at the cell surface. The immunodominance of the A36 complex in Johne's disease was suggested by comparative ELISA analysis of infected bovine sera, using either A36 or M. paratuberculosis total soluble sonicate as antigens. The cross-reactivity of TMA complexes from different mycobacteria was evaluated by immunoenzymometric measurements. Percentage of shared epitopes was high for the couple M. paratuberculosis-M. avium, and somewhat lower for the couple M. paratuberculosis.-M. bovis. Immunological kinship between M. paratuberculosis and M. leprae was suggested by the finding that out of eleven anti-M. leprae monoclonals, four cross-reacted with A36 proteins. The specificity missing at the level of the whole A36 complex was sought at the level of its protein components. Comparative immunoblot analysis of electrophoresed A36 proteins indicated three of them to contain epitopes not shared by M. bovis proteins, and one of them to contain epitopes specific with respect to M. avium, M. bovis and M. phlei. The latter component, a 34-kDa protein, could be an ideal reagent for a serological test for Johne's disease, being immunodominant in infected cattle and endowed with species-specific epitopes.