We evaluated 123 formalin-fixed, paraffin-embedded samples, including neuroendocrine tumors, adult brain, mesonephric tissues, and from various other sites. A pre-B lymphoma cell line, Daudi, and a small cell carcinoma cell line, NCL-H128, were evaluated by Western blot. All tissues were immunostained by mouse monoclonal anti-Pax-5 antibody by using standard, synthetic polymer-based detection methods. Our study describes for the first time distribution of Pax-5 in adult brain tissue, including periaqueductal gray matter of the midbrain, area postrema of the medulla oblongata, and occasional cells of the spinal trigeminal nucleus (caudal nucleus). We confirm that Pax-5 is expressed regularly in poorly differentiated neuroendocrine tumors but never in well-differentiated classic carcinoid tumors. Pax-5 expression also was found readily in benign and malignant mesonephric tissues and focally in müllerian duct-derived tissues and tumors. Expression of Pax-5 in the Daudi and NCL-H128 cell lines was confirmed by Western blot. Together, these results are important for correct interpretation of results in immunophenotyping of undifferentiated tumors, for diagnosis of mesonephric carcinoma, and, potentially, for correct classification of neuroendocrine tumors in small biopsy samples.