This chapter discusses purification and assay of CD45 of an integral membrane protein-tyrosine phosphatase. The determination of the amino acid sequence of protein-tyrosine phosphatase (PTPase) 1B, a major low-molecular-weight PTPase isolated from human placenta, has demonstrated that this enzyme is not structurally related to the protein-Ser/Thr phosphatases. CD45 represents a family of high-molecular-weight (180k–220k) integral membrane proteins, the expression of which is restricted to cells of the hematopoietic lineage. The molecule can be defined in terms of three segments. The intracellular segment is highly conserved between isoforms and comprises two homologous domains of ∼300 residues, each structurally equivalent to one PTPase molecule. The intracellular and extracellular segments are connected by a single transmembrane hydrophobic stretch of 22 residues. A simple procedure for the purification and assay of CD45 is described in the chapter.