To investigate the early spread of bovine leukemia virus (BLV) infection in vivo, we enumerated infected mononuclear cells that could express the BLV genome in vitro as they appeared in the peripheral blood of lambs newly injected with the virus. Cells that transcribed viral RNA within a few hours of isolation and cells that produced infectious virus in culture were first detected in very small numbers. Soon afterward, cells that expressed BLV transiently increased to represent 0.2 to 1.5% of the mononuclear cells. The increases occurred within leukocyte populations of normal size and cellular composition. Then, throughout the rest of the first 8 months, sharply reduced numbers of cells transcribed BLV or produced virus. All the infected animals tested by in situ hybridization displayed increased numbers of cells that transcribed BLV RNA, but only two-thirds had large increases of cells that produced infectious BLV in culture. In addition, BLV-transcribing cells exceeded virus-producing cells at most times after infection. These results demonstrate that transient increases of circulating, expression-competent cells characterize the first 3 to 4 months of BLV infection and that the extent of BLV genome expression by cultured mononuclear cells can differ among animals.