We report that complexes of giant DNA molecules with histone H1 proteins form a pearl necklace-like structure when the complexes are prepared by natural dilution from a high-salt solution (2 M NaCl) to a low-salt solution (0.2 M and 50 mM NaCl). We performed real-time observations on the conformational changes of individual T4 phage DNA (166 kb) molecules in bulk solution by fluorescence microscopy. To identify H1-binding regions on individual DNA molecules, we also performed immunofluorescence microscopic observations on the DNA-H1 complex spread on a glass surface. It was found that histone H1 binds DNA in a highly co-operative manner and is accompanied by local folding of the DNA. On the basis of the experimental observations and a theoretical simulation, we propose a self-assembling mechanism for the pearling structure.