Paramyosin fibrils from the adductor muscles of Venus mercenaria are soluble above neutrality at relatively high ionic strength. From this viscous solution it is possible, by reduction in ionic strength, to reprecipitate acicular crystals of paramyosin. In the electron microscope these fibrils manifest a symmetrical band pattern similar to that previously described by Hodge but differing in some details. The axial periods observed under the conditions of the experiment varied between 1700 and 2000 A and a simple band pattern of one-fifth the main period was frequently observed. ATPase activity of the myosin type but of much lower intensity was demonstrated. Tryptic fission of the protein occurs but the characteristics differ from those of myosin.

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