In a series of 72 open brain tumor operations the 32P (radiophosphorus) test proved to be valuable for the removal of visually not detectable tumor residues. As during resection a topographical orientation in the depth of tumor cavities was nearly impossible, a computerized measuring device was developed which enables the surgeon to locate the tumor boundaries by simultaneous comparison with preoperative CT scans. For 32P-beta emission measurements a miniature semiconductor probe can be attached to the tip of the four-axis digitizing arm allowing the evaluation of radicality by topographic distinction between tumor ramifications and normal brain tissue. The device was tested during 3 craniotomies and the perspectives of computerized stereotaxy are discussed.