A recombinant baculovirus expressing the A antigen (A Ag) of Marek's disease virus (MDV) was constructed. In Spodoptera fruiperda (Sf) cells infected with the recombinant virus, A Ag expression was localized to the cell surface. Only a small amount of recombinant A Ag was detected in the culture supernatant of infected Sf cells, but authentic A Ag is mainly secreted into the culture supernatant of MDV-infected chicken embryo fibroblasts. Cell surface-associated recombinant A Ag seemed to be slightly larger than authentic A Ag, whereas the secreted recombinant A Ag seemed to be smaller. The recombinant A Ag was shown to be reactive with the sera of MDV-infected chickens by immunodiffusion studies and ELISA. Sera of chickens immunized with recombinant A Ag formed a precipitin line with the culture supernatant of MDV-infected chicken embryo fibroblasts in an immunodiffusion test. These results indicate that the recombinant A Ag expressed by the recombinant baculovirus retains the antigenic and immunogenic properties of the authentic A Ag.