Abstract: The ultrastructure of porcine kidney (PK)-15 cells was examined after lipofectamine transfection of the molecular clone of the P1 agent. PK-15 cells transfected with the tandem dimmer of the P1 molecular DNA clone had numbers of intracytoplasmic inclusions, and a few cells had intranuclear inclusions. Intracytoplasmic inclusions were round to oval and 0.1-0.3 μm in diameter, and intranuclear inclusions were more electron dense than intracytoplasmic inclusions and were of two general types: The first type were round and small (0.1 μm approximately) and the second type were hexagon and larger (0.4-0.8 μm in diameter). The cells transfected with the tandem dimmer of the P1 molecular DNA clone were positive for P1 DNA at passages 5. This is the first report that the P1 molecular clone has infectivity in vitro and it will provide fundamental materials for further study on the biological characterization of P1.