Fifteen primary intracranial reticulum cell sarcomas and five cases with an additional solitary extracranial tumor mass have been studied. For comparison, seven extracranial malignant non-Hodgkin lymphomas and normal lymphoid tissue were included. The methods used on formalin-fixed paraffin-embedded tissue sections were an immunoperoxidase technique for the demonstration of intracellular immunoglobulins, microglial staining, Gomori's reticulin, methylgreen-pyronin, Giemsa, diastase resistant PAS, Mallory's PTAH and H&E. Electron microscopy was performed in one primary brain tumor. According to histopathologic criteria all tumors could be classified as malignant non-Hodgkin lymphomas, predominantly of the pleomorphic immunocytic or of the immunoblastic type; follicular lymphomas were notably absent. In all cases intracellular immunoglobulins were demonstrable in tumor cells and in a majority of the tumors these were monoclonal. Thus, all malignant lymphomas proved to be of B cell origin with demonstrable cytoplasmic immunoglobulin production. Based on the microglial staining more than half of the malignant lymphomas could also be classified as microgliomas. As a comparable staining was present in non-Hodgkin lymphomas outside the CNS, microglioma characteristics are not associated with intracranial growth.