Glomerular capillary filtration barrier characteristics are determined in part by the slit-pore junctions of glomerular podocytes. Protein tyrosine phosphatase receptor omicron (PTPro) is a transmembrane protein expressed on the apical surface of podocyte foot processes. Tyrosine phosphorylation of podocyte proteins including nephrin may control the filtration barrier. To determine whether PTPro activity is required to maintain glomerular macromolecular permeability, albumin permeability (P_alb) was studied after incubation of glomeruli from normal animals with a series of monoclonal and polyclonal antibodies. Reagents included monoclonal antibodies to rabbit and rat PTPro and polyclonal rabbit immune IgG to rat PTPro. The mAb 4C3, specific to the amino acid core of PTPro, decreased its phosphatase activity and increased Palb of rabbit glomeruli in a time- and concentration-dependent manner. In contrast, the mAb P8E7 did not diminish phosphatase activity and did not alter P_alb. Preincubation of 4C3 with PTPro extracellular domain fusion protein blocked glomerular binding and abolished permeability activity. In parallel experiments, P_alb of rat glomeruli was increased by 2 mAbs (1B4 and 1D1) or by polyclonal anti-rat PTPro. We conclude that PTPro interaction with specific antibodies acutely increases P_alb. The identity of the normal ligand for PTPro and of its substrate, as well as the mechanism by which phosphatase activity of this receptor affects the filtration barrier remain to be determined.