Immune response to the p43 protein encoded by IS900 was examined looking at sera IgG and cell mediated response judged by gamma-IFN release in a whole blood assay (CSL). Serological responses were assessed by comparison with a test panel of sera used for validation of the CSL Johne's disease ELISA kit (kindly supplied by Dr. S. Jones, CSL). No correlation was seen indicating that this antigen is poorly recognized at the antibody level. Whole blood, for gamma-IFN release assay, was obtained from calves in a controlled study conducted at the Victoria Institute of Animal Science (kindly supplied by Dr. A. Hope, Dr. W. McDonald and Dr. R. Condron). Groups included 6 calves naturally and 8 calves artificially exposed to M. paratuberculosis infection; 4 calves run in isolation (as controls for infection); 9 calves vaccinated with a commercial vaccine; 3 calves run as controls for the vaccinates; one mature cow as an external control. The principle interest was in the gamma-IFN release assay. Responses were seen in 8 of the 14 animals exposed to infection. Responses were seen in only 3 of 9 vaccinates. However 1 of the 4 control calves showed a response to p43 as did 1 of the 3 controls for the vaccinated group. All animals were subjected to postmortem histopathology to establish disease status. The full inference forthese results awaits complete histopathology data. Current interpretation suggests that this antigen may make a useful component of a diagnostic reagent but is unlikely to be sufficient on its own.