High concentrations of okadaic acid, sufficient to inhibit phosphatase 1 and 2A activities, induces formation of diplochromosomes in HeLa cells. It has been shown that this is due to a failure of sister chromatid separation in earlier mitosis in the presence of okadaic acid in the medium and not due to bypassing of mitosis (endoreduplication). Moreover, it has been demonstrated that the sister chromatid adherence does not depend on any under-replicated chromatin segment shared by the sister chromatids which might happen in okadaic acid induced premature mitosis, but due to the failure of the centromeres to separate at metaphase - anaphase transition. The role of phophatase 1 in sister chromatid separation has been discussed