Cytochemistry and immunohistochemistry of mast cells in the skeletal muscle of a normal BALB/c mouse (a, b) and the jejunal epithelium of a T spiralis–infected BALB/c mouse (c, d). Serial sections were evaluated for chloroacetate esterase enzymatic activity (a, c), immunoreactive mMC-CPA (b), and immunoreactive mMCP-2 (d). The arrows indicate mast cells that appear to possess multi-lobular nuclei. Panel (a) was heavily counterstained with methyl green. Thus, the three chloroacetate esterase⁺ mast cells depicted in this panel exhibit a blue color. The red reaction product on the brush border of the villi (d) is due to endogenous intestinal alkaline phosphatase. The presence of this product indicates that the color substrate was active in the immunohistochemical reaction.