Summary The production, purification and properties of the ‘A’ antigen induced by Marek's disease virus in tissue culture were investigated. It was found that although virus multiplied on passage in chick kidney, chick embryo and duck embryo cells the antigen was produced only in chick kidney and duck embryo cultures. The results suggested that the ‘A’ antigen produced by duck embryo cells was a glycoprotein having a mol. wt. in the range 70 000 to 90 000 and was heterogeneous in charge (pI 4.5 to 5.5). A 20-fold purification was achieved by electrophoresis in 5% acrylamide gel with a recovery of 45%. An equally good purification was also possible by chromatography on DEAE-Sephadex A50 and Sephadex G200, although the recovery was only 20% in this case.