Recently proposed image comparison software is applied to immunoscintigraphy. The software performs geometric and gray level registration of two images and generates an image of the statistically significant differences. It permits the comparison of scintigraphic images recorded at different times. It is used to subtract 113mIn and 111Inphytate colloid liver scans and early (blood pool) images from 131I or 111In-monoclonal Ab images and to compare Ab images recorded at different times. Using the procedures made possible by this software, only images recorded using the same radionuclide or using radionuclides of about the same energy are compared. Anti CEA, 19-9 and OC 125 F(ab)2 fragments labeled with 131I or 111In are used in 32 patients with 47 demonstrated recurrences or metastases of colorectal or ovarian cancers. The overall sensitivities of the unprocessed and processed images are 25/47 and 41/47 respectively. The improvement in sensitivity is particularly high in the liver when In labelled Ab are used. This technique improves the contrast of the images, but the interpretation must take into account the components of the non target activity (kidney, bone marrow, colon...) which are not removed by the image subtraction method.