Maedi Visna is a chronic contagious infectious disease of small ruminants caused by Maedi Visna Virus (MVV), and characterised by lymphoproliferative inflammation in lung and udder. T regulatory cells (TReg), including CD4+CD25+ cells, have been recently demonstrated to be involved in immunomodulation of chronic disease. In order to contribute to the present knowledgements, a preliminary immunohistochemical study was performed to detect the CD25 antigen in pulmonary and mammary sections of sheep experimentally infected with MVV and necropsied after three years. The number of CD4CD25+ cells could be correlated with the type of the lymphoproliferative lesions.