Oligonucleosomal DNA fragmentation and chromatin condensation are two hallmarks of apoptosis. However, their generation mechanisms are not entirely understood. Histone H1, a positively charged nuclear protein located in the linker region of chromatin, is involved in higher-order chromatin structures and tight chromatin packing. On the basis of the physical and biochemical characteristics of histone H1, we hypothesized that histone H1 plays a role in determining the efficiencies of apoptotic DNA fragmentation and chromatin condensation. Therefore, we examined histone H1 quantity in five human leukemia cell lines and compared the efficiencies. The cell lines were categorized into two groups according to their origins: (i) Ramos and Molt-4 cells of lymphoid origin and (ii) U937, ML-1, and HL60 cells of myeloid origin. Compared to the lymphoid-origin group, the myeloid-origin group had lower levels of histone H1 but more open chromatin. Furthermore, the myeloid-origin group showed marked DNA fragmentation but less chromatin condensation during apoptosis. These results suggested that histone H1 determined chromatin structure and that its quantity affected the efficiencies of DNA fragmentation and chromatin condensation in apoptosis.