The function of the phosphorylation of histone H3 at Ser 10 in plant cell division is uncertain. The timing correlates with chromosome condensation, and studies in plant meiosis suggest that it is involved in sister chromatid cohesion. In mitosis, plant chromosomes are highly phosphorylated in the pericentromeric region only. In order to modulate H3 phosphorylation, root meristems of different plant species were treated with the protein phosphatase inhibitor cantharidin or with ice-water. Immunostaining using an antibody specific to phosphorylated H3 at Ser 10 revealed a high level of H3 phosphorylation along the whole mitotic chromosome after cantharidin treatment, which resembles the distribution seen exclusively in first meiotic division. In chromosomes that were isolated from meristems treated with ice-water, the heterochromatic regions and nucleolar organizer regions, in addition to the pericentromeric region, were highly phosphorylated at H3. Cantharidin and ice-water also affected spindle assembly and chromosome length, but these effects did not seem to be directly linked to changes in H3 phosphorylation.