A monoclonal antibody against the surface marker IOT-10 of natural killer (NK) cells was used to investigate the presence and distribution of these cells in a series of nine primary intracranial germinomas. In all of these tumors, IOT-10-positive NK cells were found in small numbers, mainly distributed among the tumor cells. The data obtained in the present study suggest that the presence of NK cells in primary intracranial germinomas can be influenced by factors other than the mere quantity of tumor-infiltrating lymphocytes.