Electron microscopy of negatively stained preparations of phosphorylase b with protamine added reveals three types of crystal formations: plane monolayers of particles, tubes and three-dimensional crystals. The methods of optical diffraction and filtering have been used to analyse the micrographs. The unit cell of the layer contains two packing units. Most of the tubes have one-and two-layer walls corresponding to monolayers of particles, but sometimes tubes occur with three and more layers in the wall. Optical diffraction shows that the packing units in tubes with a one-layer wall form a two-start helix in which 27 units fall on 3.5 turns. The inner tube in tubes with two walls has an analogous structure. The outer tube represents a three-start helix in which 19 packing units fall on 5 3 of a turn. The unit cell in the layers forming the wall contains one packing unit. The analysis of the image of particle packing-units shows that these are the same in the layer and tube walls. Particles of the same type were found in preparations of phosphorylase b tetramers from solution. A model for the particles is proposed; it consists of four elongated, bent subunits arranged with the point-group symmetry 222 at the vertices of a tetrahedron. One has good grounds for considering the observed particles to be the phosphorylase b molecules in the form of tetramers.