In a study on leucocytic movement, it was found that leucocytes showed periodical dynamic patterns with each motile function, and a possible organization in their motile system. It was also clarified that the motile form and function of leucocytes were co-ordinately controlled by the intracellular level of ATP and that the characteristic contraction wave observed in moving leucocytes was substantial as a morphological manifestation of the contractile element in moving leucocytes. Based on these findings, an attempt was made to extract contractile protein from leucocytes. It was shown that the protein consisted mainly of myosin and actin, which is similar to protein of muscle. Thus, it was concluded that development of the pseudopod, which is indispensable for cell movement, seemed to result from liquid substance in the granuloplasm being squeezed out through contraction of contractile protein located in the surface layer of the granuloplasm. In non-muscular cells, the same type of ordered structure as seen in muscle has not been found yet, but it seems likely that the protein is capable of converting chemical energy into movement.