Probe counter examinations of brain tumor suspects having received 1.5-2.0 mc of radioactive P the previous day either by mouth or vein has repeatedly been proven to be advantageous in locating, delineating and removing these tumors. In addition equivocal lesions have their presence actually established and a pathological diagnosis is obtainable in almost every instance of expanding intracranial mass. Sufficient evidence is also presented to indicate that this procedure can be used in the general practice of neurological surgery. In contrast to other counting methods, the probe counter procedure is not obligatory at any certain time but may be varied according to the demands on the neurosurgeon.