Previous studies have identified a group of cells in the dorsolateral hypothalamus that project to many different areas in the CNS, such as thalamus, diagonal band of Broca, basal ganglia, cerebral cortex, hippocampus, and olfactory bulb. Their role is presently unknown, but the cells have been reported to stain for an intriguing array of putative neurotransmitter-related substances, including a-melanocyte-stimulating hormone (aMSH), melanin-concentrating hormone (MCH), human growth-hormone-releasing factor 1-37 (hGRF 1-37), corticotropin-releasing factor (CRF), metorphamide, and acetylcholine esterase. A monoclonal antibody produced in the present study, aC11, stains both the cell bodies of this system in hypothalamus, with a punctate pattern, and varicose fibers in the various target areas. In double-label immunocytochemical experiments in rat DLH, aC11 and MCH staining exactly overlaps. Concentrations of aMSH and MCH high enough to completely block staining with the corresponding antisera had no effect on staining with aC11. Similarly, CRF, hGRF 1-37, and metorphamide were unable to block a C l l staining. The results suggest that the antigenic epitope for a C l l is not contained in aMSH, MCH, CRF, hGRF, or metorphamide, and thus, that a C l l is detecting another antigen uniquely expressed in these neurons. The punctate appearance of staining in the hypothalamus and the concentration of staining in fiber varicosities suggests that the a C l l epitope may be involved in synaptic function.