Serine/threonine protein phosphatase 2A (PP2A) may play a role in leukaemic cell differentiation of the HL-60 myeloid leukaemic cell-line after methylprednisolone induction. We have investigated the specific enzyme activity and expression of catalytic and regulatory subunits of PP2A. The resulting specific enzyme activity and immunoblots showed an increase in enzyme activity and the expression of regulatory subunits after methylprednisolone treatment. There was no change in the expression of PP2A catalytic subunits. It is suggested that the effect of methylprednisolone on leukaemic differentiation may be the result of PP2A upregulation.