Immunofluorescence microscopy: PCI in washed platelets. Smears of washed platelets isolated from whole blood were incubated with a monoclonal antibody against PCI (A), PAI-1 (B), factor D (C), or heparin cofactor II (D); in other negative control experiments with dilution buffer alone (E); or with a nonimmune mouse IgG (F), followed by fluorescein-conjugated goat-antimouse-IgG. PCI showed fluorescence intracellularly as well as PAI-1, which was used as a positive control (A and B, respectively), whereas a rim pattern of staining consistent with an exclusively surface localization of the protein, which was observed with the antibody to factor D (C), was not observed in case of PCI. Negative control experiments performed with an antibody to heparin cofactor II (D), with mouse-nonimmune IgG (F), or by incubation with dilution buffer alone (E) did not show any fluorescence. (Original magnification × 3,000; exposure time: 3 seconds.)