Leukocyte suspensions derived from genetically Marek's disease (MD)-resistant N-line and MD-susceptible P-line chickens were fractionated at various times after exposure to the JM-10 clone of MD virus. At 3 and 5 days post exposure (DPE), during the productive-restrictive (cytolytic) phase, most infected spleen and thymus leukocytes were found to be low-density, nylon wool-adherent cells that possessed Fc receptors and surface Ia and IgM and were depleted by carbonyl iron treatment. This was true for leukocytes derived from N-line as well as those from P-line chickens. In contrast, most infected spleen cells derived from P-line chickens during the latent phase (i.e., after 7 DPE) were not found to have the above characteristics, with one exception: Ia antigen was demonstrated on the surface of latently infected cells. From these experiments it was concluded that the principal targets of the cytolytic JM-10 infection are B-cells, whereas the subsequent latent infection was found mostly in non-B-lymphocytes.