To clarify the biological activity of the cloned provirus genome of a retrovirus produced in a human lymphoblastoid cell line, the cloned genome was introduced into cultured cell lines by DNA transfection techniques, and the viral gene products were analyzed. The transient expression of virus-related antigens was detected immunocytochemically in a canine thymus cell line two days after the introduction of the cloned provirus genome. Stable transfectants of a human lymphoblastoid cell line (Jurkat) were established by introducing recombinant molecules constructed with the provirus genome and neomycin resistance gene and by selecting with G-418. Southern blotting analysis showed that the full-length provirus genome was integrated into these cellular DNA. The provirus genome-containing transfectants were shown to express 35S RNA, which was thought to be viral genome RNA, by Northern blotting analysis. Western blotting analysis showed that the viral major structural protein p34 and p34-related proteins were expressed in these transfectants. These data indicate that this cloned provirus genome encodes viral RNA and proteins and suggest that this provirus genome could be useful for the introduction of various genes and the analysis of their expression in human lymphoblastoid cell lines.