Abstract:The highly vigrous subprotoplasts were prepared from the germinated pollen of Lilium. As the protoplasm mass contracted, many cytoplasmic fibrils with free-ends which moved like animal sperm tails appeared at the surface of the mass. The winding movement of the fibril could tow the free-end‘s cytoplasm mass, but did not affect the particles moving along the fibril. Only when the fibril free-end adhered to the inner side of the cell membrane, could the, particle movement along the fibril occur, with the disappearance of the fibril’s winding movement. In vitro, the fibril contraction could make both cytoplasmic particles and subprotoplast move in unidirection, and the fibrils could specifically bind fluorescent beads coated with rabbit myosin. This indicates that the fibrils were composed of F-actin. We think that the cytoplasmic streaming may be based on the contraction of F-actin which must adhere to some points of the inner side of the cell membrane, and the contraction of F-actin drives the membrane-bound organells to move, at the same time, propels the sol cytoplasm thus forming the cytoplasmic streaming observed by light microscopy.