Sap-I/PTPRH, a receptor protein tyrosine phosphatase (RPTP), is a ubiquitously expressed enzyme that is upregulated. in human gastrointestinal cancers. Using both chemical cross-linkers and co-immunoprecipitation we show that overexpressed full-length SapI is present as a stable homodimer. Unlike a number of adhesion RPTPs which have tandem catalytic domains that are involved in dimerization, Sap-1 has a single catalytic domain, and we show that this domain is not required for Sap-1 dimerization, which is mediated instead by the large extracellular and transmembrane domains. Exposing cells that express the receptor to a reducing environment reversibly disrupts the Sap-1 dimer, suggesting that cysteine bonds play a role in dimer formation/stabilization. The switch between Sap-1 dimers and monomers is accompanied by an increase in catalytic activity as judged by its capacity to dephosphorylate and activate c-src, which we identify as a novel substrate for this phosphatase. © 2005 Elsevier Inc. All rights reserved.