The localization of 113mIn-ethylenediamine tetraethylenephosphonate and 113mIn-diethylenetriamine pentamethylenephosphonate in myocardial infarct induced in rabbits by coronary artery ligation was investigated. Double labeling of the same tissue with 45Ca ion was used as an index of tissue degeneration. The radioactivity ratio (40-50:1) of infarcted to normal myocardium was obtained 1.5 hours after injection and 2.5 hours after infarct. For the 45Ca-labeled phosphonate derivatives, a ratio of 7-6:1 was obtained at the same time. These agents will thus be useful for the detection and localization of acute infarct.