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A structural comparison of two subtypes of histone 1 (HI) in HeLa S-3 cells (H1A and H1B) shows that the cationic C-terminal half of HI A is smaller and more hydrophobic (containing ca. seven more valyl residues) than the comparable region of H1B, suggesting that the two proteins differ substantially in their interactions with DNA, proteins, or both. Differences between the N-terminal halves of HI A and H1B were found only in the short region between residues 16 and 30. H1B was found to have more phosphorylation sites than HI A. For location and comparison of phosphorylation sites within HI A and H1B during the S phase and mitosis, cells at these two stages were incubated with [32P]orthophosphate, and tryptic peptide maps of the TV-bromosuccinimide fragments were examined. In S-phase cells, most of the 32P incorporated was at two places in the C-terminal cationic tail of each subtype. Experiments with hydroxyurea at the time of the Gj-S transition suggest that the onset of phosphorylation at one of these sites at that time does not require a substantial amount of DNA replication but that the onset of phosphorylation at the other site does. Phosphate groups at the former type of site were shown to be of shorter durationIn the preceding paper (Ajiro et al., 1981), we showed that the HeLA histone 1 (HI)1 fraction contains two principal subtypes. H1A and H1B, which differ in molecular weights and have different phosphorylation levels at all stages of the HeLa S-3 cell cycle. HI A is predominantly unphosphorylated in G,, monophosphorylated in the S phase, and tetraphosphorylated during mitosis. In contrast, H1B is predominantly monophosphorylated in G! and diphosphorylated in the S phase and appears to have five to eight phosphate groups in mitosis. These striking and different changes in HI A and H1B phosphorylation levels during G,, the S phase, and mitosis