Technetium-99m-labeled pyrophosphate and radiolabeled antimyosin antibodies are two infarct localizing agents with apparently different kinetics of localization. To determine whether these agents localize in a similar fashion in early acute myocardial necrosis, we studied the simultaneous distribution of 111In-labeled antimyosin and 99mTc-labeled pyrophosphate in dogs after intracoronary (i.c.) (n = 9) or intravenous (i.v.) (n = 9) administration of a mixture of these two agents in a reperfused infarct model. The mean infarct size (+/- s.d.) delineated by pyrophosphate [20.2 +/- 14.1 (i.v.), 29.8 +/- 12.3 (i.c.)] was larger than that by antimyosin [14.2 +/- 11.3 (i.v.) (p = 0.05), 20.0 +/- 11.8 (i.c.) (p = 0.05)] which was larger than that by triphenyl tetrazolium chloride [13.9 +/- 8.0 (i.v.) (p = 0.05), 15.3 +/- 6.5 (i.c.) (p = 0.05)]. This overestimation persisted whether the radiopharmaceuticals were administered by intracoronary or intravenous injections, although the latter with antimyosin was only slightly larger (TTC:AM = 13.9:14.2) (p = n.s.). There was a good correlation, however, between antimyosin and pyrophosphate delineated infarct sizes in dogs with intracoronary injection (y = 0.82x + 13.33, r = 0.79) or i.v. injection (y = 1.208x + 3.01, r = 0.97) of the mixture of the two agents. Since the images of the 111In and 99mTc activities were obtained consecutively by identical methods, the overestimation of infarct size by pyrophosphate cannot be due to differences in spatial resolution of the techniques used. The differences in the areas of myocardial damage delineated by pyrophosphate and antimyosin in our study most probably denote the area of viable but compromised myocardium.