Several single-cell clones were isolated from a fetal lamb kidney cell line (FLK) persistently infected with bovine leukemia virus (BLV). The majority of isolated cell clones were virus productive, several were nonproductive based on the determination of the activity of reverse transcriptase and production of 3H-uridine labeled virus particles. Two nonproductive clones, NP-1 and NP-2, were further characterized in comparison with the virus productive cells. All clones contained three integrated BLV proviruses in the nonproducer cells. The virus specific mRNAs were expressed both in the virus productive and nonproductive cells. The virus-specific protein products were found different in the nonproducing cells. The gag pol precursor Pr145 was missing in NP-1 cells, in NP-2 its Mr was 120 only. In NP-2 cells the precursors Pr70gag and Pr45gag were absent. The env precursor gPr72 and both of the two glycoproteins were detected. The nonproductive cells NP-2 produced mainly env gene products, therefore they were tested as a potential material for anti-BLV vaccine. The NP-2 cells after inoculation to rats and cattle were able to induce formation of neutralizing antibodies directed against the env gene products.