An immunohistochemical analysis of Rhodococcus equi-induced pneumonia in 10 foals was performed by biotin-streptavidin system. The detection of R. equi was more sensitive in immuno-stain using anti-R. equi serum than in Gram's stain. This bacteria also reacted to anti-BCG serum. Lysozyme and alpha 1-antitrypsin were detectable in macrophages. A particularly intense staining was observed in association with intracellular bacteria. Though a degree of reaction for alpha 1-antichymotrypsin was very low in comparison with lysozyme and alpha 1-antitrypsin, it was also demonstrated in macrophages ingesting R. equi. These bacteria were almost intact under an electron microscope. Therefore, the surface components of R. equi may play important roles of protection from intracellular enzymes of macrophages. The cells containing intracytoplasmic IgM, IgG or IgA were a few in number and scattered predominantly around the pneumonic lesion. It is considered that the bactericidal activity by immunoglobulins may be weak in comparison with phagocytosis by macrophages.