Distinct inositol and phosphatidylinositol polyphosphate 5-phosphatases have recently been cloned. Primers were designated coding for highly conserved amino acid regions that are shared between sequences of 5-phosphatases. We used degenerate primers to amplify polymerase chain reaction products from rat brain cDNA. A product with a novel sequence was identified and used to clone a 4.9 kb cDNA from human placenta cDNA libraries (hp51CN). COS-7 cells transfected with a C-terminal truncated form of this cDNA showed an increase in Ins(1,3,4,5)P4 and PtdIns(3,4,5)P3 hydrolyzing activity, but not in Ins(1,4,5)P3 5-phosphatase. Enzymatic activity was inhibited in the presence of 2,3-bisphosphoglycerate and p-hydroxymercuribenzoate. The presence of an SH2 domain and proline-rich sequence motifs within hp51CN suggests that this 5-phosphatase interacts with various proteins in signal transduction.