The stage of activation of bronchoalveolar lavage fluid (BALF) lymphocytes and peripheral blood lymphocytes (PBL) from maedi-visna virus (MVV) infected (n = 7) and control (n = 7) sheep was investigated by assessing four parameters of lymphocyte activation; lymphocyte size and complexity, loss of CD5+ T cells, expression of cell surface interleukin-2 receptor (IL-2R) and expression of DR and DQ MHC Class II molecules. BALF lymphocytes from MVV-infected animals had a significant loss of CD5+ lymphocytes (P < 0.05) and upregulation of DR and DQ MHC Class II molecules compared with controls, consistent with BALF lymphocyte activation. No changes in cell size and complexity or expression of IL-2R were observed. No evidence of PBL activation was detected. These findings suggest an impaired BALF lymphocyte activation during MVV infection.