SUMMARY A phenolypic analysis of bronchoalveolar lavage fluid (BALF) and peripheral blood (PB) cells in maedi visna virus (MVVVinlecled sheep has been performed. The differential cell eount in BALF from MVV-infeeted animals was characterized by a significant increase (P < 0±05) i n lymphocytes and neutrophils. Lymphocyte phenotyping in BALF from MVV-infeeted sheep showed a significant decrease (P < 0·05) or CD4+ cells, a significant increase (P < 0·05) of CD8+ eells and a significant inversion (P<0·001) of the CD4+/CD8+ ratio. CD5+ lymphocytes were also significantly decreased (P < 0·05). γδ T cells and B cells did not differ significantly when compared with the controls. No correlation was observed between BALF and PB lymphocyte phenotypes. BALF macrophages from MVV-infeeted animals showed inereased MHC class II expression and BALF lymphocytes from the same animals demonstrated up-regulation of LFA-1 and LFA-3 expression. These findings and their relationship with lentiviral pathogenesis are discussed.