Both intact and nuclease-isolated chromatin fibers have been examined at different degrees of salt-induced compaction, using a variety of preparation techniques. The results suggest that the initial folding step in nucleosome packing involves the formation of a zig-zag ribbon as has been proposed by others (Thoma F., T. Koller, and A. Klug, 1979, J. Cell Biol., 83:403-427; Worcel A., S. Strogartz, and D. Riley, 1981, Proc. Natl. Acad. Sci. USA, 78:1461-1465), and that subsequent compaction occurs by coiling of the ribbon to form a double helical structure. This type of folding generates a fiber in which the nucleosome-nucleosome contacts established in the zig-zag ribbon are maintained and in which the histone H1 molecules occupy equivalent sites. The diameter of the fiber is not dependent upon the nucleosome repeat length. Direct mass values for individual isolated fibers obtained from electron scattering measurements showed that the mass per length was dependent on ionic strength, and ranged from 6.0 X 10(4) daltons/nm at 10 mM NaCl to 27 X 10(4) daltons/nm at 150 mM salt. These values are equivalent to 2.5 nucleosomes/11 nm at 10 mM NaCl and to 11.6 nucleosomes/11 nm at 150 mM salt and are consistent with the range of packing ratios for the proposed helical ribbon.