99Tcm-DTPA has been evaluated in our clinical and experimental programme for the detection of acute gastrointestinal bleeding. As an adjunct to this programme, a protocol for image subtraction has been developed. The patient remains still while sequential static images I(i) (i = 1, . . ., N) are taken. They are first normalized to equal total counts and then subtracted images are produced according to the following three methods (a) I(i + 1)-I(i) (b) I(i) - I(mask) (c) I(mask) - I(i) where i not equal to mask and I(mask) denotes a user-selected mask image. Method (a) demonstrates fresh bleeding and sequential movement of blood in the bowel. Methods (b) and (c) demonstrate overall migration of blood and accumulated bleeding depending on the choice of the mask image.