Is there a third type of filament in striated muscles?

Using glycerinated rabbit psoas myofibrils we demonstrate by electron microscopy that there is a filamentous material which remains after extraction of the fibrils with a modified Hasselbach-Schneider solution (containing 1.0 M KCl) followed by extraction with 0.6 M KI. The fibrils were visualized by conventional negative staining techniques using either 2% ammonium molybdate or 1% uranyl acetate. The appearance of the extracted fibrils together with their solubility properties strongly suggests that the extraction-resistant filamentous material is composed of connectin, an elastic protein recently characterized from muscle tissues.