The 75-kDa (type II) inositol polyphosphate-5-phosphatase, originally described in platelets, is one of at least three known enzymes capable of dephosphorylating inositol-1,4,5-trisphosphate (IP3) to inositol-1,4-bisphosphate (IP2). To further characterize these enzymatic forms, we have mapped the gene (INPP5B) coding for the 75-kDa type II enzyme. Using a combination of human x rodent somatic cell hybrids and fluorescence in situ hybridization, we have determined that this gene maps to human chromosome band 1p34.