Light microscopic immunohistochemistry based on the principle of capillary action staining is a widely used method to localize antigens. Capillary action immunostaining, however, has not been tested or applied to detect antigens at the ultrastructural level. The aim of this work was to establish a capillary action staining method for localization of intracellular antigens, using colloidal gold probes.Post-embedding capillary action immunocytochemistry was used to detect maternal IgG in the small intestine of newborn suckling piglets. Pieces of the jejunum of newborn piglets suckled for 12 h were fixed and embedded into LR White resin. Sections on nickel grids were secured on a capillary action glass slide (100 μm wide capillary gap, Bio-Tek Solutions, Santa Barbara CA, distributed by CMS, Houston, TX) by double sided adhesive tape. Immunolabeling was performed by applying reagents over the grids using capillary action and removing reagents by blotting on filter paper. Reagents for capillary action staining were from Biomeda (Foster City, CA). The following steps were performed: 1) wet the surface of the sections with automation buffer twice, 5 min each; 2) block non-specific binding sites with tissue conditioner, 10 min; 3) apply first antibody (affinity-purified rabbit anti-porcine IgG, Sigma Chem. Co., St. Louis, MO), diluted in probe diluent, 1 hour; 4) wash with automation buffer three times, 5 min each; 5) apply gold probe (goat anti-rabbit IgG conjugated to 10 nm colloidal gold, Zymed Laboratories, South San Francisco, CA) diluted in probe diluent, 30 min; 6) wash with automation buffer three times, 5 min each; 7) post-fix with 5% glutaraldehyde in PBS for 10 min; 8) wash with PBS twice, 5 min each; 9) contrast with 1% OSO4 in PBS for 15 min; 10) wash with PBS followed by distilled water for5 min each; 11) stain with 2% uranyl acetate for 10 min; 12) stain with lead citrate for 2 min; 13) wash with distilled water three times, 1 min each. The glass slides were separated, and the grids were air-dried, then removed from the adhesive tape. The following controls were used to ensure the specificity of labeling: i) omission of the first antibody; ii) normal rabbit IgG in lieu of first antibody; iii) rabbit anti-porcine IgG absorbed with porcine IgG.