Neutron scattering in solution and scanning transmission electron microscopy were simultaneously done on chicken erythrocyte chromatin at various salt and magnesium concentrations. We show that chromatin is organized into a higher-order structure even at low ionic strength and that the mass per unit length increases continuously as a function of salt concentration, reaching a limiting value of between six and seven nucleosomes per 11 nm. There is no evidence of a transition from a 10-nm to a 30-nm fiber. Fiber diameter is correlated with mass per unit length, showing that both increase during condensation. We also find that there is no essential difference between the mass per unit length measured by scanning transmission electron microscopy and neutron scattering in solution, showing that the ordered regions seen in micrographs are representative of chromatin in solution.Nucleosomes in chromatin interact to form a higher-order structure, referred to as the 30-nm filament. Despite a large number of structural studies on chromatin, there is no unanimity on its higher-order structure. The nature and sources ofconflicting views ofthe structure ofthe 30-nm fiber have been summarized (1).Several models of the 30-nm fiber propose a helical structure for chromatin. These include the solenoidal model (2), the helical-ribbon model (3,4), the crossed-linker doublehelical model (5), and the nonsequential single-helix model (6). All of these models share certain similarities; for example, they all satisfy constraints imposed by electric dichroism measurements on the angle of the nucleosomal disk with respect to the fiber axis (7,8). Most ofthe models have similar repeat dimensions that would show up as peaks in a diffraction pattern. The helical-ribbon model predicts certain additional peaks due to the width of the ribbon. These additional peaks have not been observed in an experiment on oriented chromatin (9), but it is possible that an adjustment in the parameters of the helical-ribbon model would eliminate the additional reflections that are predicted.Despite such similarities, the models differ considerably in detail. Even at a coarse level, the various models predict different values for the mass per unit length of condensed chromatin. Thus a measurement of the physical characteristics of chromatin condensation could help to substantiate or rule out some of the existing models for chromatin structure.The mass per unit length of chromatin has been measured by small-angle scattering (10-14). The parameter has also been measured by hydrodynamic methods (7, 15) and light scattering (16). These experiments generally yield a value of 6-7 nucleosomes per 11 nm for the mass per unit length of the condensed 30-nm fiber. However, scanning transmission electron microscopy (STEM) studies on chromatin (4, 5) have yielded much higher values, of up to 11.6 nucleosomes per nm. Felsenfeld and McGhee (1) have pointed out that the discrepancy may be due to the fact that in STEM studies one selects ordered regions of chromatin, wh...