Although well established myocardial infarcts (MI) are not difficult to identify in routine hematoxylin and eosin stained sections, recent MI may present diagnostic problems. Fibrinogen is a useful marker for detecting early ischemic cell damage. Using immunofluorescence on frozen tissue, albumin, IgG and fibrinogen have been found throughout the sarcoplasm of ischemic fibers in human hearts. In this report, monoclonal antibodies to all three proteins were reacted using the avidin-biotin technique in formalin fixed, paraffin embedded sections from autopsy cases of sudden or intraoperative deaths with either subtle or no definite ischemic changes evident in routine sections. Strong staining of fibrinogen in the fibers associated with coagulative necrosis, contraction bands or wavy fibers, and in the fibers presumably associated with acute ischemia. Albumin and IgG staining was nonspecific. Fibrinogen is a reliable and reproducible marker for recognizing early ischemia. This method can be used to diagnose early sudden ischemic and intraoperative deaths due to coronary artery bypass graft and prosthesis-related complications and may be particularly useful for forensic autopsies.