In mammals there are two forms of alkaline phosphatase, one of which is widely distributed in a variety of tissues, and one of which is confined to intestine. Levamisole (1-tetramisole) inhibits the nonintestinal form of the enzyme, but is without effect on the intestinal form. We have exploited this difference by using conjugates made with calf intestinal alkaline phosphatase for immunohistochemical demonstration of H2 antigens in frozen section of mouse tissues. The alkaline phosphatase staining is performed in the presence of 1 mm levamisole, which inhibits the endogenous tissue enzyme without loss of staining by the conjugate. Endogenous enzyme can be inhibited by other means, such as exposure to 20% acetic acid, but labile antigens may be destroyed.