The bovine leukemia virus (BLV), like the human T cell leukemia viruses (HTLV-I and -II), is usually present in its host in a transcriptionally repressed state. However, the viral genome becomes derepressed a few hours after the infected lymphocytes are cultured in vitro. Depending upon the concentrations tested, plasma and lymphatic fluid of BLV-infected cattle have either stimulatory (BSF) or inhibitory (PBB) activity on viral expression in these cultures. These activities can be separated by chromatographic procedures. BSF is either an antiviral antibody or a BLV-induced molecule that binds to IgG. After complete removal of BSF, the PBB activity can be more consistently detected in bovine plasma and lymphatic fluid. PBB activity can also be demonstrated in human plasma. It seems likely that this activity is responsible for the latent state in which BLV, HTLV-I and -II, and human immunodeficiency virus are usually present in their hosts.