Agar gel precipitin (AGP) antibody was detected in 31 of 283 (11%) pheasant (Phasianus colchicus) serum samples using antigen prepared with spleen tissue from a pheasant dying of marble spleen disease (MSD). These same serum samples failed to react with normal pheasant spleen or antigens of chick embryo lethal orphan (CELO) virus or Marek's disease virus.