Abstract: The specific activity is an i mportant index of radioiodine-labeled neuro-receptor imaging agents. If no stable nuclide is introduced in the course of preparation, the specific activity of the product is dependent on the specific activity of radioisotope source. However the unlabeled precursor in the final product may reduce the effective specific activity of the products unless it has been removed,, Thus we suggest to use "apparent specific activity" instead of "specific activity". Using "A_(SA)", we could approximately evaluate the effect of unreacted precursor on the specific activity of products. In the preparation of neuro--receptor imaging agents it may be very useful.