The nuclear sub-structures known as ND10, PODs or PML nuclear bodies can be rapidly modified by diverse stimuli, and the resultant structural changes correlate with events such as cellular transformation and successful virus infection. We show that the ND10 components PML and Sp100 undergo profound biochemical changes during the cell cycle. Both proteins are conjugated to the ubiquitin-like protein SUMO-1 during interphase, but they become de-conjugated during mitosis and an isoform of PML of distinct electrophoretic mobility appears. This mitosis-specific form of PML is highly labile in vitro, but is partially stabilised by phosphatase inhibitors. Treatment of interphase cells with phosphatase inhibitors induces the production of a PML isoform of similar gel mobility to the mitosis-specific species, and taken together these results suggest that phosphorylation is an important factor in the differential modification of PML during the cell cycle. PML and Sp100 normally tightly co-localise in ND10 in interphase cells, but they become separated during mitosis. Interphase cells treated with phosphatase inhibitors or subjected to heat shock also show structural changes in ND10, accompanied by alterations to the normal pattern of PML modification. Taken with previous findings on the effects of infection by herpes simplex virus and adenovirus on ND10 structure and PML modification, these results suggest that the many factors which have been shown to modify ND10 structure may do so by interaction with the biochemical mechanisms that act on ND10 components during the cell cycle.