Japanese quail were inoculated with Marek's disease herpesvirus at 1 day of age, and a second group of the same age was infected by contact with inoculated quail. A third group raised in our laboratory as a source of hatching eggs was accidentally infected with Marek's disease (MD). All quail were observed and examined for periods up to 360 days; the usual clinical signs of MD were not observed. However, the birds were susceptible to MD infection because of the evidence that: 1) The MD-specific agar gel-preceipitation (AGP) antigen was found in the feather tips or feather follicular epithelium of some of them; 2) the MD-specific AGP antibody was found in 6 of 57 exposed to MD; and 3) the virus was recovered from 4 MD-exposed birds by subpassage of their kidney cultures into quail embryo fibroblasts. The specificity of the AGP antigen or antibody in MD-infected quail was examined by the line of identity and cross-absorption tests. The antigen specificity was tested against similar to 700 sera from chickens and quail. Although most of these sera reacted with quail-positive antigen (QPA) prepared from MD-infected quail, some sera from infected or uninfected normal chickens reacted nonspecifically with QPA and with an antigen prepared from normal quail. By gel filtration on Sephadex G-150, the specific antigenic component could be separated from the nonspecific component(s) in QPA.