Abstract Immunological stimulation of rat mucosal type mast cells (line RBL-2H3) by clustering the type I Fc ɛ receptor (Fc ɛRI) causes a fast and transient tyrosine phosphorylation of several proteins. This implied the involvement of both, protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPases) in that process. In order to identify the PTPases involved in these very early steps coupling Fc ɛRI stimulus to cell response, we undertook the purification and characterization of PTPases present in RBL-2H3 cells. In one of the cells' membranal fractions, a PTPase activity was found to be enhanced 2- to 3-fold upon cell stimulation by Fc ɛRI clustering. Characterization of this activity implies its involvement in control of the FC ɛRI signalling cascade.