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.