The protein phosphatase activity in rat liver cytosol or nuclear extracts that dephosphorylates histone H1 which has been phosphorylated by p34cdc2 is inhibited completely by okadaic acid, but unaffected by inhibitor-2 or magnesium ions, demonstrating that the only enzyme in this tissue capable of dephosphorylating this substrate is a type 2A phosphatase. Fractionation of the cytosol by anion-exchange chromatography and gel filtration demonstrated that histone H1 phosphatase activity coeluted with the major species of protein phosphatase 2A, termed PP2A1 and PP2A2. PP2A1 was the most active histone H1 phosphatase, its histone phosphatase phosphorylase phosphatase activity ratio being 6-fold higher than PP2A2 and 30-fold higher than the free catalytic subunit PP2AC. It is concluded that PP2A1 is likely to be the enzyme which dephosphorylates p34cdc2-labelled histone H1 in vivo and that the A and B subunits which interact with PP2AC in this species each play a key role in facilitating dephosphorylation of this substrate. The results demonstrate that PP2A, in addition to being involved in suppressing the activation of p34cdc2 in vivo, can also function to reverse at least one of its actions.