Twenty-eight oligodendrogliomas and seven oligoastrocytomas were immunotested by the peroxidase-antiperoxidase (PAP) method with antiglial fibrillary acidic protein (GFAP) serum, anti-Leu 7 monoclonal antibody (Mab), anti-myelin-associated glycoprotein (MAG) Mab, anti-myelin basic protein (MBP) serum, anti-carbonic anhydrase C (CA C) serum and anti-neuron-specific enolase (NSE) serum. The immunoreactivity of their vascular pattern was studied with Ulex europaeus type I lectin (UEA I). According to their morphology and distribution GFAP-positive cells were respectively interpreted as reactive astrocytes, neoplastic astrocytes and neoplastic oligodendrocytes. Reactive astrocytes were found in the tumor, around the tumor and surrounding the supporting blood vessels. Neoplastic astrocytes were mainly found in the oligoastrocytomas and usually closely intermingled with neoplastic oligodendrocytes. GFAP-positive neoplastic oligodendrocytes were found in the typical oligodendrogliomatous areas. They had central nuclei and GFA positivity was mainly found in the perinuclear cytoplasm. They correspond to the "gliofibrillary oligodendrocytes" described by Herpers and Budka. Of the oligodendrogliomas 91% displayed Leu 7 positivity, but anti-Leu 7 cannot be considered as a specific marker for oligodendrogliomas since other neuroepithelial tumors have been reported to react with this antibody. MAG-, CA C- and NSE-positivities were found in a number of tumor cells in a few oligodendrogliomas. All the tumor cells were MBP-negative, but myelin sheaths and fragments of myelin in the infiltrated white matter were clearly demonstrated by this antiserum. UEA I strikingly demonstrated the vascular pattern of the tumors, and its usefulness as a discriminating marker for the supportive endothelial cells was confirmed.