Cultures of dispersed spleen cells, prepared from MDV-infected chickens with MD visceral lymphomas, showed marked depression of responsiveness to the T cell mitogen PHA, as measured by 3H-Tdr incorporation in cells in vitro. When data are expressed quantitatively in terms of cpm/105 viable cells, the functional depletion of PHA-responsive cells appears to result from lower levels of 3H-Tdr incorporation in the PHA-stimulated spleen cultures from chickens with acute MD symptoms, as compared to similar cultures from uninfected isolator-reared control chickens. It is suggested that depression of PHA-induced blastogenesis in spleen cell cultures from chickens with acute MD reflects virus-related alterations in T lymphocytes.