The expression of gag (p15, p25) and env gene products in ovine lentivirus-infected cells was studied in 20 adult Texel ewes seropositive to maedi-visna virus and 10 seronegative matched controls. Bronchoalveolar lavage was performed to recover alveolar cell pools from which cytocentrifuge preparations were made. Single and double immunocytochemical techniques were applied to study viral replication and coexpression of viral markers with markers for macrophages, lymphocytes, and major histocompatibility complex (MHC) class II. Aveolar macrophages of eight of 20 infected sheep (40%) were positive for viral protein expression. The percentage of positive macrophages varied from < 1% to 12% of the total population of macrophages. Viral protein expression was not detected in lymphocytes or other cell types. A relationship between virus-replicating macrophages and differential expression of MHC class II molecules, upregulated in ovine lentivirus infection, could not be established. Pathology was evaluated in nine infected ewes. Animals with the highest levels of positive cells had moderate or severe lymphoid interstitial pneumonia. However, sheep with similar degrees of lesions had lower percentages of positive macrophages or were negative for viral protein detection. These results support the idea that a partial or even a complete loss in the restriction mechanism of maedi-visna virus in lungs can occur in some individuals.