Mason-Pfizer monkey virus-related antigen was detected in 3 out of 5 jaagsiekte lungs examined using a direct immunoperoxidase staining technique with anti-MPMV p27 serum. Most of the antigen was localized in the alveolar lumina of the lesions. The reaction was further characterised on immune blots and found to involve a protein with a molecular mass of 29 000 daltons (JSRV p29). JSRV p29 antigen was also detected in 2 jaagsiekte cell lines.