The catecholaminergic neurons of the nervous system have been studied histochemically with fluorescent derivatives of catecholamines and immunocytochemically using antibodies against their biosynthetic enzymes. The immunocytochemical techniques yield permanent preparations and make possible ultrastructural studies and combined applications with other procedures. In this report, we describe the production and application of a high-affinity mouse monoclonal antibody against the rate-limiting enzyme in the biosynthetic pathway of the catecholamines, tyrosine hydroxylase. This antibody, coded TOHA1.1, has been used successfully to stain tyrosine hydroxylase immunoreactive sites in the known catecholaminergic neurons and fiber systems of rat brain in both light and electron microscopy. It has also been demonstrated that TOH A1.1 will immunoprecipitate phosphorylated tyrosine hydroxylase.