While it has been suggested that JC virus (JCV) migrates in B-lymphocytes from the kidney to the central nervous system where it initiates demyelination, this phase of JCV pathogenesis has not been systematically explored. To determine the peripheral blood cell subpopulation(s) infected with JCV, monocytes, granulocytes, and T and B lymphocytes from HIV-1-infected individuals and uninfected controls were purified by flow cytometry. JCV DNA could be detected by PCR amplification in all of these cell subpopulations. This finding suggests that JCV lacks specificity in its interaction with leukocytes.