The success of radioimmunoguided surgery (RIGS) is dependent on the performance parameters of the apparatus, its correct handling, and the employment of a suitable radioactive compound. In the present study the authors examined the performance of the Neoprobe 1000 device with respect to the radioisotope 99mTc. Detecting efficiency was evaluated experimentally using a phantom containing a radioactive point source which could be moved both vertically and horizontally with respect to the central axis of the probe. In this way curves representing the variations in efficiency as a function of the vertical and horizontal distances of the source from the probe were constructed. Furthermore, values of minimum detectable activity as a function of source depth, counting time and background radioactivity were calculated. These results were compared to those previously obtained using 125I, the radioisotope most frequently employed in RIGS. The graphs and tables included could serve as a practical aid to help the operator obtain the best possible measuring conditions, thereby maximizing his/her results.