HeLa cells treated for prolonged period with okadaic acid (OA; 5-10nM) inhibiting protein phosphatase 2A (PP2A) and also protein phosphatase 1 (PP1) partially showed prolonged effects on mitotic progression. In the presence of OA cells progressed normally in mitosis almost upto 4 hr, then a progressive accumulation of mitotic cells could be noticed. Most of the mitotic cells seemed to be arrested at the metaphase-anaphase transition point. In arrested mitotic cells the chromosomes remained arranged at the equiatorial plate, but with prolonged treatment the chromosomes got either scattered or clumped. However, a slow release into anaphase could also be observed after 15 hr treatment. Immunofluorescence studies for microtubules and electron microscope investigations indicated the dearrangement of spindle fibres, and a prolonged treatment led to the formation of multipolarity. This was also confirmed by spread preparations of chromosomes and the formation of multinucleate cells in preparations released from the mitotic block. Chromosomes became highly condensed showing mostly nondisjunction, but separation of sister chromatids could be observed in many cells. Immunoblot assays indicated a degradation of cyclin A, but the cyclin B1 level was significantly higher in the arrested mitotic cells after 12 hr treatment. After 24 hr of treatment the cyclin B1 level was slightly lower in arrested cells. Possible roles or protein phosphatase 2A inhibition and a prolonged partial inhibition of PP1 on the mitotic progression and the cyclin degradation at the metaphase-anaphase transition have been discussed.