Immune serum was prepared in the rabbit with BAI strain A leukosis virus isolated by centrifugal fractionation from the plasma of chickens with myeloblastic leukemia and further purified on a potassium tartrate gradient. Antibody to groupspecific antigen was demonstrated in the serum by immunoelectrophoresis and immunodiffusion. Fluorescein-conjugated serum was used unabsorbed and absorbed with chick cells for study of acetone-fixed chick embryo cells uninfected or infected with strain MC29

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... n leukosis virus. With unabsorbed serum, large numbers of cytoplasmic particles stained in a few cells within 2 hr after exposure to virus, and the cell number increased greatly in 24 hr. Absorption of the serum abolished the early reaction. Staining with absorbed serum was delayed until about 14 hr after culture exposure to virus, but essentially all cells were stained within 72 hr at the time when all cells were morphologically altered. Differences between the responses to unabsorbed and absorbed serum suggested cytoplasmic formation or concentration of chick tissue antigen similar to that incorporated in leukosis virus particles. The characteristics of staining with absorbed serum were similar to those observed by others in analogous studies with avian tumor viruses. Strain MC29 (12) avian leukosis virus induces chiefly (12, 19) myeloid hematopoietic tissue tumors or diffuse growths of myelocytes in the chicken. The virus and growths contain group specific (GS; 11) and other antigens common to the avian virus-tumor complex as demonstrated (9) in cross-reactions by gel immuno-precipitation, neutralization, and complement fixation tests.