Visna virus is the prototype of the lentivirus subfamily of retroviruses that cause slow infections of sheep and goats. These viruses persist and can be isolated from blood and cerebrospinal fluid for years despite neutralizing antibody. In the studies reported here we have used quantitative in situ hybridization to analyze infected leukocytes. We show that (1) monocytes harbor the visna genome; and (2) virus gene expression is as constrained in this cell as it is in glial and epithelial cells. These results are in accord with a Trojan Horse mechanism of virus dissemination in an immunologically responsive host.