Cyclophilin A is a small, soluble protein which binds the immunosuppressive drug cyclosporin A. Cyclophilin A is a peptidyl-prolyl isomerase and is widely expressed in a multitude of tissues, but is present in highest concentration in the brain. A role for this protein in the maturation and folding of neuron-specific membrane proteins has been hypothesized. Immunohistochemical staining for cyclophilin A was used to determine whether cyclophilin A is present in neurons, and whether there is variation in the level of expression with respect to brain regions and cell types. The specificity of the antibody used was demonstrated by Western blot analysis and cyclosporin affinity purification. Immunohistochemical staining of sections of adult rat brain showed labelled neurons throughout the neuraxis. The intensity of the immunostaining observed was roughly equivalent to neuronal cell density and was restricted to gray matter. On a cellular level, staining was present in cytosol and nuclei and extended into neuronal processes. Fluorescent double-labelling experiments on hippocampal cell cultures revealed that all cells labelled with the cyclophilin A antibody also showed staining for the neuron-specific marker for microtubule-associated protein (MAP)2, and could, therefore, be identified as neurons. Immunoreactivity in these neurons is present in punctate, spinelike structures along dendrites. Cyclophilin A immunoreactivity was undetectable in glial fibrillary acidic protein (GFAP)-positive cells. The pattern of cyclophilin. A expression is consistent with a role of cyclophilin A in neuronal protein maturation and folding in vivo.