The amino acid sequence of a novel mammalian protein phosphatase, termed PPX (and designated PPP4 in the human genome nomenclature), has been deduced from the cDNA and shown to be 65% identical to PP2A alpha and PP2A beta and 45% identical to PPI isoforms, the predicted molecular mass being 35 kDa. PPX was expressed in the baculovirus system. Its substrate specificity and sensitivity to the inhibitors, okadaic acid and microcystin, were similar (but not identical) to the catalytic subunit of PP2A. However, PPX did not bind the 65 kDa regulatory subunit of PP2A. The intracellular localization of PPX was investigated by immunofluorescence using two different antibodies raised against bacterially expressed PPX and a PPX-specific peptide. These showed that although PPX was distributed throughout the cytoplasm and the nucleus, intense staining occurred at centrosomes. The centrosomal staining was apparent in interphase and at all stages of mitosis, except telophase. In contrast, antibodies directed against bacterially expressed PP2A were not specifically localized to centrosomes. The human autoantibody #5051, which stains the pericentriolar material, colocalizes with PPX antibodies, suggesting that PPX may play a role in microtubule nucleation.