Peripheral blood lymphocytes from bovine leukemia virus (BLV) infected cattle were grown in vitro with serums from BLV-infected and uninfected cattle, sheep and rabbits. Only serums from infected animals which had antibody to gp 45/55, the major glycoprotein antigen, inhibited the release of virus from the cells. Although viral antigens could be detected in the cells themselves, none were detected in the supernatants of cultures grown with these serums. Normal serums and serums with antibody only to p23, the internal BLV antigen, did not inhibit virus release. To identify the factor responsible for virus release inhibition, 2 inhibitory serums were absorbed with either gp 44/55, p23 or tissue culture cells from BLV-infected and uninfected cells lines. The results indicated that antibody to gp 45/55 is the factor responsible for virus release inhibition.