Whether the early infarct area in formalin-fixed, paraffin-embedded tissue could be delineated by the immunohistochemical method using myoglobin-antibody was studied in 23 pig hearts without collateral circulation. Five hearts were examined at 20 minutes, 2 hours, 4 hours, and 6 hours after occlusion of the distal one third of the left anterior descending coronary artery, respectively. Three pigs were killed 24 hours after occlusion. Heart rate and aortic pressure before and after occlusion did not change in any groups. The subepicardial and subendocardial regional blood flows were reduced to almost zero in all hearts after occlusion (0.88 +/- 0.10 to 0.02 +/- 0.02 mL/g/min). Slight myoglobin defects in the ischemic tissue were noted in the five pigs examined 2 hours after occlusion and definite myoglobin defects were detected in all pigs examined at 4, 6, and 24 hours after occlusion. Nitrotetrazorium blue stain of myocardial tissue before formalin fixation showed slight demarcation of the ischemic area at 4 hours after occlusion and definite demarcation at 6 and 24 hours after occlusion. Slight demarcation was noted at 2 hours after occlusion in Masson trichrome stain and 4 hours after occlusion in the hematoxylin-eosin stain. However, definite demarcation of the ischemic area was seen in Masson trichrome stain only at 24 hours after occlusion and was not noted in hematoxylin-eosin stain even at 24 hours after occlusion. Our previous electron microscopic study revealed that, in the pig heart, irreversible cellular damage was transmurally seen at two hours after occlusion of the coronary artery. Therefore, a definite myoglobin defect reflects irreversible cellular damage such as infarction.(ABSTRACT TRUNCATED AT 250 WORDS)