Abstract The mast cell function associated antigen (MAFA) is a membranal glycoprotein identified on the surface membranes of rat mucosal-type mast cells of the RBL-2H3 line by a monoclonal antibody (G63) binding to it. MAFA clustering by mAb G63 causes a dose-dependent inhibition of these mast cells' response to immunological stimulus provided by the type 1 Fcε receptor (FcεRI) suppressing the biochemical processes coupling it to mediator secretion. The inhibition was found to take place upstream to the production of inositol phosphates and the transient increase in free cytosolic Ca 2+ ion concentration, hence it probably interferes with the cascade at the level of the protein tyrosyl kinases (PTK) activity. We have therefore examined whether MAFA clustering affects protein tyrosyl phosphorylation of cell components and found that a time-dependent increase is caused in this modification of the FcεRI-β chain. This constitutes the first evidence for the capacity of the clustered MAFA to enhance, on its own, biochemical changes in the mast cells, changes that are most probably related to its inhibitory signaling capacity. Moreover, that the observed phosphorylation changes are in the FcεRI-β chain clearly indicates possible cross-talk between these two membrane components.