A method was developed for the large-scale production of Marek's disease herpesvirus A antigen in duck embryo fibroblast roller bottle cultures in quantities sufficient to permit its purification and characterization. Maximum yield was obtained in serum-free culture medium harvested daily. The Marek's disease herpesvirus A antigen was stable at pH 2.0 and was a glycoprotein based on its sensitivity to trypsin, specific immune co-precipitation of radioactive amino acids and glucosamine, and detection of radioactive glucosamine by immunodiffusion and autoradiography. The antigen aggregated and lost titer upon storage but dissociated readily and regained titer in 1 or 2 M urea and 0.05% Brij 35. Fresh unaggregated antigen or antigen dissociated with urea and Brij 35 sedimented at 3.7S on sucrose gradients. The apparent molecular weight of the glycoprotein antigen was estimated to be 44,800 by gel filtration on Sephadex G-200 in the presence of 2 M urea and 0.05% Brij 35.