Hematopoietic destructive lesions, characterized by necrosis and loss of architecture in the bursa of Fabricius and thymus, were observed consistently in second-generation specific-pathogen-free X Line 7 (SPF X L7-F2) cross chickens inoculated at 1 day with an acute form of Marek's disease (MD) agent. In addition, there was a drastic reduction in packed cell volume and lack of hematopoiesis. Serologic examination of the dams of these chicks revealed an absence of antibody to MD-associated herpesvirus. The incidence of hematopoietic destructive lesions was reduced when genetically similar chicks from artificially immunized antibody-positive dams were inoculated with acute MD agent. The unusual response in chicks inoculated with acute MD is thought to be explainable on the basis of varying degrees of passive protection through maternal antibody. Chicks without such protection are more prone to develop aplasia of bone marrow and necrosis of bursal and thymic lymphoid elements.