A study of the susceptibility of human peripheral blood mononuclear cells to measles virus infection and replication is reported. Resting lymphocytes obtained from adults showed very low levels of infection and virus replication while lymphocytes activated by plant mitogens or allogenic lymphocytes supported mononuclear cells obtained from the umbilical cord of healthy neonates were more susceptible to measles virus infection than those of adults; however, activated cord lymphocytes supported viral replication in the range observed with adult activated lymphocytes. Monocytes obtained from adults were relatively resistant to measles virus infection and replication while neonatal cord blood monocytes supported viral replication to the degree observed with activated lymphocytes. It is hypothesized that infection of acitivated lymphocytes may explain the depression of cell-mediated immunity seen during acute measles virus infection. The significance of the finding that neonatal monocytes are more susceptible to viral infection and replication than adult monocytes is discussed.