Sera from 3 cows with the adult form of lymphosarcoma inhibited release of leukemia virus from a cell line of fetal lamb spleen infected with bovine leukemia virus (BLV). Sera from 5 or 7 cattle experimentally infected with BlV also suppressed virus release. The inhibition of virus release was reversible. Sera from cattle with the calf form and the thymic form of lymphosarcoma and normal bovine control sera did not repress virus release.