Immune suppression has long been recognized to be a consequence of measles and a likely contributor to the secondary complications of this infection. Since measles virus can be isolated from peripheral blood leukocytes during the early phases of infection, it has been widely assumed that measles virus suppresses the immune system by replicating in and damaging T lymphocytes. A reverse transcriptase-polymerase chain reaction (RT-PCR) assay able to detect wild-type strains of measles virus was applied to the study of peripheral blood mononuclear cells (PBMC) during natural measles virus infection. Measles virus RNA was detected by RT-PCR in monocyte- enriched adherent cells up to 6 days after the appearance of the rash. No viral RNA was detected in the lymphocyte-enriched nonadherent cell fraction. The numbers of virus-infected PBMC detected by in situ hybridization ranged from 1 in 125 to 1 in 2500. Dual staining identified infected cells as monocytes. It is concluded that the primary leukocytes infected during measles are monocytes, not lymphocytes. This cellular tropism has implications for understanding the immune suppression associated with measles.