Subcellular localization of PR3 by electron microscopy on neutrophils thin frozen sections labeled with anti-PR3. Electron micrograph of resting neutrophils from the same individual stained for localization of PR3 with the MoAb anti-PR3 CLB 12.8 followed by incubation with 10-nm gold particles-conjugated goat antimouse (GAM 10). (a) Gold-labeled antibody is present at the periphery of large extracted granules identified as the azurophil granules (ag) as well as in the membrane of intracellular empty vesicles (v). Plasma membrane labeling is shown by arrow heads (original magnification × 35,200). (b) The immunogold label (arrowheads) indicates the presence of PR3 on the plasma membrane in a random distribution head (original magnification × 42,550).